Simply put, the most cost-efficient way to get exposure and downloads for your iPhone app is by building a great viral loop into your product. Regardless of your audience or interaction, there is some kind of growth-loop available to you that by doing well, is going to be far more cost-effective than any of the other answers provided so far.

Paid acquisition, PR, really anything other than WOM (almost impossible to come by) or virality (which should be possible if your core audience cares about what you're doing) are going to be cost-inefficient relative to the competition for attention you're fighting against.

I'm happy to look at your product and in a quick call brainstorm some potential growth loops with you.