Often in games there’s a regular currency and a more premium one. Usually completing tasks and challenges give you the regular currency, and leveling up/purchasing gives you the premium.
I haven’t personally used this economy style but it seems to be popular and used quite often.
I’ve found a really good video about MMO economy a while back, maybe it helps you designing your economy.
If you’re considering using a form of currency to purchase in-game content, whether that’s free (earned via natural progression/play-time) or premium (purchased with Robux), you’ll want to carefully consider exchange rates and accrual rates.
You’ll want to properly gauge content by how much they should cost in Robux, and then price it accordingly with your in-game currency. This will prevent hyper-inflation which could ruin your in-game economy and mess with the overall perception of the game.
Premium Currency
The goal of premium currency is to set a “premium” or price on in-game advantages. It shouldn’t be cheap, to prevent players from reaching the end-game prematurely. It shouldn’t be too expensive or else you won’t sell your currency. When selling currency to players, you’ll want them to feel like they got their money’s value, so if it costs 100 “gold” to buy the lowest level item in a game, you don’t want to price it at 10,000 robux for example. When the prices are too high, that’s a deterrence for your players. They won’t buy something they don’t see as worth their money. Keep in mind that overly-generous rates will also have an adverse effect. If it costs 10,000,000 “gold” to buy the best item in the game, it shouldn’t cost 10 robux.
Free Currency
Similar to premium, you don’t want players prematurely reaching the end-game. So the rates that players accrue free currency should be fair but low, as the free option should be slower than the premium option. You have the power to decide how much time it should take players to progress, and controlling rates of free currency accrual is a part of that.
Some game producers limit the types of things premium currency can buy, to prevent imbalances to the game. The famous “pay-to-win” complaint. Limiting premium currency purchases purely to “aesthetic content” such as weapon skins, emotes, trails, lights, things along those lines, are popular choices when developers wish to keep game-progression natural and untainted. However, selling in-game advantages, which can often be branded as “pay-to-win” are likely to sell more frequently as they’re a more attractive purchase. You should carefully consider what you allow your players to buy and set prices accordingly to preserve the game’s progression so people play in ways you intend.
You won’t always be able to control players, their perception of your game, or how they play your game, but thinking about how you setup your economy can help corral your players to behaving in predictable ways, which can help you plan updates and changes accordingly.
I have watched this, it is helpful but my game doesn’t have Player-Player shop so it won’t inflate.
My main idea around figuring this out is how much money is given to a player for how much time played and how much do items cost.
Very informative 
My game will have a few types of currencies
Money - Earnable
Diamonds - Premium & Earnable
So my current plan is you get Money every Level and then every 5 levels you get more Money and some Diamonds.
You also earn Money from Daily rewards and participating in In-Game Activities, Quest, Missions, you name it.
That is what I am trying to figure out, the Ratio of how much play time for x amount of Money + Item Pricing
Items need to be priced in a way that a player has an incentive to keep playing the game and not get bored of playing it because the item is taking way to long to get the money to buy it.
That is what I’m having trouble with, I don’t know the ratio of that and how long between each payout should be or how much money it should be for players to be comfortable, I’m trying to figure out the sweet spot.
Not too slow and not too fast.
I would say looking at a few games, the first few “items” don’t take long to get the money for then as you level up in your case items should take a little longer to farm for.
Doubtful you will find the sweet spot during the planning phase. Probably best to develop a hypothesis (best guess) about how things should go, and then play test (with other people… friends or whatever) to find the sweet spot.
Building off of what waterrunner said, you might consider playing your own game and trying to see how long it takes to earn certain milestones. Having Alpha/Beta testers is great for this.
A lot of studios, in real life, and on Roblox, bring in testers to see if progression needs tweaking. Even triple A studios have to adjust things before full-release. So early-access testers are the people you ask to help you check if things are too grindy or if things are too easy.
Building on what you have just said, games aren’t perfect and games are always changing the item prices to balance the game out, even the best games. If an item is too cheap the developers will adjust the price slightly even the when game is released and out of beta.
Where do you get Testers if you aren’t using Roblox QA Tester Team
You will need to use your own team of testers if it is your game as the QA testers test roblox features. You could ask your friends to test the game or try and hire people to test it.
Once again building off of what waterrunner said, you can also ask for DevForums members to test your game. You can simply make a thread, introduce your game, and ask for feedback. Devs can offer in-depth criticism and analysis that regular players lack.
Note: I can’t remember which subcategory you go to post a thread asking for game-feedback.
Here are a few tips and tricks I have learned throughout the years:
Make sure the game economy is balanced. If anything try and make it challenging for someone to earn items, to increase player retention. Players who want to keep playing to get a reward using the in game currency is a great way to use economy to benefit a game. However don’t make it to the point where it’s impossible to get items.
Besides having a currency that is balanced you should also ensure you have a currency that means something. People have to want to earn the currency whether it’s for weapons, cosmetics etc.
Make a currency that doesn’t devalue. Every time a player gains currency it adds more money to the game which eventually means a worthless currency. A great way of removing that issue is if things need repairing or a price for dying etc. Or make things that are consumable such as drinks and potions which mean nothing, and help remove currency from the game.
If you can create a currency that is balanced, means something and doesn’t devalue you will have the perfect economy.
NOTE: In terms of pricing which is an issue your having, think about how much money people get over time, and decide how long you want them to wait to get a certain item.
I find that rather than using a 2 currency economy, having a singular currency is very effective.
Make the more basic items a user will purchase early on fairly cheap, and from there raise prices.
Once they are “hooked” on your game they will make larger currency purchases and buy more expensive and adventagious products.
While opinions may differ, something I like seeing in games is currency purchasing as a percentage of your current balance. If there is donating in the game then this will probably lose its value, however it otherwise helps prevent players from buying their way along the game journey too far. You also ensure that purchases have the same value no matter where in the game the player is. This method is best for games like tycoons, simulators, and similar games.
As said in many of the above posts, simplicity is really important. At all costs avoid more than two currencies. I personally do not have a preference between one or two currencies, however it should be noted that monetization becomes more obvious to the player when you have a premium currency mainly or completely earned with Robux. An example of where too many currencies can cause issues is the somewhat new game Brawl Stars by Supercell. (Famous for Clash of Clans and Clash Royale) They have several currencies-experience, coins, gems (premium currency), tokens, large tokens, token doublers, league, trophies per hero, total trophies, hero levels, and power points. A few of these may mot be currencies, however they represent someone’s ranking and are important. Although they did a good job on keeping it easy to understand, having all of these numbers to keep track of can become confusing and is support for keeping everything simple. If the currency is going to be complex, balance it with a simple level system so that it is easy for the player to manage. I find it important to think about how many numbers and systems the player needs to think about in general instead of just focusing on categories like currency.
Gifting would be through Robux only so they don’t inflate and cause problems with making new account just to exploit the system.
As for my game I do have Premium currency but they are Earnable, I want everyone to have fun in my game with or without Robux.
I know that a lot of people will never ever get any Robux so I don’t want to discourage them, would be nice if the Ad system is still here.
Another idea is giving each currency a different direct use, ala many MMORPGs. That way, it’s clear what it’s for, but the values can fluctuate.
My tips are to plan it out ask yourself these questionsQuestions! What is the genre of the game?Once you got the genre you can get your core ideas in.