Anywhere from nothing, to letting the developer monetize the result themselves, to 100+ hours at $100+/Hr, to much much more (iFund VC level salary plus stock options). The average of that wide a range won't mean much.
You'll have to convince most competent iPhone developers that your idea and remuneration rate per day will very likely be much better than their (possibly inflated) expected rate of return for developing their own app ideas and putting them into the App store by themselves.
Can you do that?
.