Well, you don't need more than 1k-2k robux for starting, then after you make a lot of money from the game you can start spending some real money like 10k-20k. Don't spend much if you don't know for sure that you will have a big audience playing your game.
Can you monetize Roblox on YouTube?
Am I allowed to monetize videos of Roblox gameplay that contain Licensed Music on YouTube? Yes. Roblox users may generate revenue through appropriate advertisements placed on Video Captures containing Licensed Music, including pre-roll ads, ad breaks, and sponsor ad overlays on YouTube.