In my experience, and keep in mind that’s exactly what it is… my POV…
Sponsors can be a good way to get an initial wave of players coming to your game. This lets you quickly see what “sticks” and what does not. As for how much you should spend, probably no less than $100 for a single advertising campaign.
To give a concrete example; one of the games I worked on a few years ago, Minion Simulator, we started with $100 advertising campaigns per day for a week. This got us a pretty good sample of users to play with, and the game generated enough revenue to keep the team working on it for about a year full time.
I have had many successful game launches over the last 10 years, but hardly ever due to Roblox platform advertising. Especially since the changes to sponsored experiences late last year; I’ve seen significantly worse returns on my spending. I hardly ever do it anymore.
What does work quite well, is the Roblox Algorithm. Roblox wants to push what they deem are good quality games. They have many metrics to look at what makes a game good quality, and if you optimize for this, your chance at a successful title will increase.
If you’re doing poorly on the algorithm, then there is an issue with your game that players are not vibing with. It could be many things. A not-so-great game loop, maybe poor user experience. Could be low server FPS, low client FPS, poor network optimization, etc.
When I optimize my games to work well with the types of things Roblox likes to see (more in game spending, more in game hours played, high amounts of returning users, etc), I see a great return in my time-investment. i.e. the time I spent making the game more enjoyable for my playerbase.
My latest project, Aftermath, we spent $0 on advertising. We grew it organically at the start by offering free weekends to players, to give them a “try before you buy” option. This worked pretty well, and we have been able to see some good growth over the last year and a half.