Completely agreed with Evan. The only time I would ask a shop to do work without money being exchanged is when they're taking equity in a project. A blended rate of $79 per hour is at the mid point of reasonableness and $120-$150 per hour would be the high point.

If you're asking about rates, it sounds like you don't have a lot of experience building mobile apps so if that's the case, I'd suggest you charge the low end of the range because it has the potential to act as a big reference account for you. If on the other hand, I'm reading the situation incorrectly, then charge whatever your normal rate is.

I refer a lot of work to a couple of mobile app shops I trust and like and have previously worked at an interactive agency. Happy to talk to you in a call to give you more perspective and clarity.

Best of luck!