I would say at most $99.
If you develop an iPhone Web Application, you don't need Apple's App Store to distribute it.
If you develop a Native iPhone Application, you will probably distribute it through Apple's App Store (unless it is limited to a small number of users). To be able to do that, you will have to enroll to Apple's developer program, which costs $99 (last time I chcked).
See more details in Native iPhone Applications vs. iPhone Web Applications.
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