In general, a good source for competitive salary data is salary.com, but they don't tend to break it out by mobile developer vs. "normal" developer.
A reliable developer with at least one year of experience generally could expect to make a full time salary in the $80,000 - $100,000 range. For Silicon Valley, add $10,000 or so. For nowhereville or Phoenix, subtract $10,000 or so.
Since mobile developers are extremely scarce right now and in very, very high demand, add about 25%.
These are full time salaries. For a single programmer working as a contractor on a 1099 basis or for short-term gigs, divide the annual salary by 1000 to get the hourly rate. For example, a high-end mobile developer in Silicon Valley might earn $130,000 as a full time rate or bill $130/hour for part time work.
I see all these huge numbers and just have to be a contrarian. If you recruit just like everybody else from same places with same spec, yes, $80K-$120K is reasonable. That is the penalty you pay for doing everything the same like the masses do.
But here in Boston, where demand is no less ridiculous than in SV, the smart ones have mobile devs in $60-$70K range. They may not be MIT CSers, but they will out-code them (cleanly and efficiently) any day, since there are no brain cycles wasted on entitlements and egos (I don't blame the MITers though, when everyone wants you, your ego explodes).
Instead, focus on finding "born to be engineers" talent. With 100+ devs I've had hand in hiring, never saw a good one who could not pick up and master any new language/platform in under a month. I personally don't care if you match my spec, I want to know if you have right fundamentals and I will invest in you in exchange for more sustainable pay.
Here in Indonesia we pay $500 per month and it's very good. That's how I become a businessman. There is no way to make a living being programmer here.
Recently I hire many high IQ programmers and I am satisfied with their skills and talents. Some is even better than I am.
I think I would subcontract jobs from American and train more programmers. I can give good portfolio, etc. Not sure how to do so though.
In my country, in Ukraine, you can hire high skilled team(5 programers) for $100/hour or make an app by a price You pay for 1 employe in USA. Try to look developers in other countries, it might be very useful.
Here in Phoenix most mobile app developers that I've come across make between $5,000 and $20,000 per project depending on the scope.