If the project has any future beyond the prototype, your friend would be crazy not to ask you to stay involved. The time to discuss compensation and risk is then. If you want something to happen, do a good job with the project and make your partners excited by the prospect of doing more.
Let them own the prototype, they are paying for it after all (however little that might be). It shouldn't be hard for you to completely rewrite the prototype if you want to go forward with the idea yourself (after all you're talking about 20 hours of initial work, and chances are the rewrite will be much faster and of much higher quality).
If the project has any future beyond the prototype, your friend would be crazy not to ask you to stay involved. The time to discuss compensation and risk is then. If you want something to happen, do a good job with the project and make your partners excited by the prospect of doing more.