The only robux you would have to spend on making a game is advertisements and if you need to hire people for specific jobs. They both have ranges, and your post isn’t really specific so I can’t give a range for hiring developers. Advertisements I recommend starting out with 10k robux on ads, sponsoring, etc. each and running for a few days and then seeing which ia received better and spending more on that one.
For more info on ads, see The Economic Guide to Profitable Game Advertising and more if you search on the devforum.