$2500 for 50 hours of work works out to $50 p/hour. I don't know what the fair market rate is for freelance client side app dev, but that sounds a bit low. Most web designers I know bill higher than that. For $50 an hour I'd either expect the developer to be really junior, or experienced but totally unaware of their own value.

Also, I agree that providing a percentage of the possible profits is not a very good motivator. Unless you're some master marketer with proven businesses experience behind you, it's a bad gamble for the programmer.