I remembered that there’s a formula for developers to calculated how much robux they pay for each job in a development team. I heard that scripter and builder are paid with lots of robux.
However, I wanna know if theres any good formula to calculate the price to pay someone robux for contributing my game. Can anyone help me?
It depends on the person you hired. If you hired someone to help make an asset, you should pay them based on each asset. If someone is long-term, you should pay them on a schedule, for example monthly, daily, etc. Truly, it depends on how much you think the person worked, what they did, what attitude they had doing it, and a bunch of other factors. However, as long as you and the person you are paying are satisfied, that’s all that matters in the end. 
It’s usually a matter of asking about their preferences. For me, this is the “formula” I usually follow
For long-term work
For short-term work
In the end, if you’re unsure and don’t feel like asking too much, just try to get your contractor to be paid by direct payments. Hope this helps! 
Regarding how much you should pay? Perhaps per asset to be safe? Never heard of this aforementioned “formula” but I tend to look at their payment rates and try to settle on something in mine and their range in terms of funds. There’s many more factors but, like @SenpaiBaconus said, as long as you and your developer are satisfied with the pay, 
The best way to pay someone robux would probably be small payments over a certain amount of time, for example.
•Tuesday-100Rb
•Wednesday-500Rb
Next week
•Wednesday-1000Rb
It makes sure you have enough to pay them while keeping them working still.