Not to mention that APPLE takes a 30% cut.
And, personally, I would NEVER hire a foreign developer to do this shit. You WANT to meet face to face, trust me. At least, it's definitely preferable. There's an element of face to face communication that you simply cannot reproduce even over Skype, etc.
I say this because this is what we did at the startup I had with two friends. You do NOT want some Chinese/Indian/foreign developer, regardless of the cheap labor (compared to here).
I would find someone with a history of mobile app development. I would personally look for a UI designer with serious back end dev experience. Typically, the front end and back end guys are separate, but they also historically clash over how they want to implement features: the designer wants it one way, the back end guy wants to implement it in such a way that "break" the design.
And no, developers will not typically agree to future payments via app sales.
IF you were to hire someone abroad, you'd want it to be a legit company with a history of development, ideally one that has dealt with American companies, so that the firm's reputation could be at stake in the event of an NDA breach. An individual could simply be anonymous and hard to pursue if you want to take legal action.
Trust me, everyone and their mother has "an app idea." I'm not saying yours isn't legit, I'm saying that it takes a MONUMENTAL effort to be the next Angry Birds or whatever app you're thinking of. It takes innumerable (seriously, stop counting) iterations and user feedback. It takes an act of god to convince devs (really, really good ones) to pour their talent into something that may not take off anytime soon.
The really good devs want to see their work take off and be used immediately. They get sore if the business dev guy/girl isn't doing his/her job; they want to see their work make an IMPACT. Plus, they want that shit for their portfolio to show off.
I would look at HOW to hire a good programmer. There are articles on this. Definitely look for people that have work on github (share their code publicly) to assess how passionate/prolific they are. That's only one way, but there are other ways.