Ask HN: How do I know what to charge?

An associate has asked me to build an iOS app for him. I am a new iOS developer and this would be my first freelance project of any kind (I am a full time Python/Django developer with 3-4 years experience). I have no clue what would be a reasonable fee to charge him. I want to skew to the low-end since I have a personal connection with him and would be learning as I go, but I want to be fair to both him and me. I would assume a flat rate is better than hourly given the slow speed with which I will program. He's looking for an app that would get a user's location in the city we are in (so, limited to just this Metro area), provide a route somewhere else via Google Maps and our local bus transit's API, and pop up with interesting trivia along the way (that he would provide me). The project seems interesting and at the right level for me, skill-wise, but I have no idea how to estimate what that's worth.