Roblox takes 75% of each sale. This means the developer needs to make 400,000 in Robux sales before the developer accumulates 100,000 Robux for cash withdrawal.
If 100,000 Robux is sold for $1,000, and 100,000 Robux only nets the developer $350, then the double dip gives Roblox a net 91.25% of developer sales. This excludes any applicable taxes or app store fees.