mfitzgerald, as others have said, there is never a guarantee of sales. If you make no money at all, with your efforts be worth it?
If the company is successful, how much you think is fair can only be decided by the two of you, yet, if you make sure that going into the project you get what you need without getting a single cent back for your time and effort, you are more likely to be happy getting what you want when and if you do get it.
Does that make sense?
For example, you are are saying this is a way to get your internship done. OK, that is one benefit to you. If that is all that happens and the company folds because it is poorly run, or the economy won`t support it, or whatever, will you still be happy with the time you spent?
If you only get 5%, and the company grows to $8million/year, is that enough, when $0 was enough before?
You see the difficulty here?
I suggest a really quick read of "Sway: The Irrisistable Pull of Irrational Behaviors." It`s not a business book, per se, but one of the chapters specifically discusses the different ways of looking at fairness, and may help you with your thought process.