I've never published an iPhone app personally, but at my last job we hired TakeFiveLabs to make an app for us (Veritas Prep GMAT app), and though it was completely free, it brought in $10,000+ in signups for our $1500+ course and tutoring
How much did you pay to have this app created? Also, by $10K in signups, does that mean seven or so signups for a $1.5K course?
They charged us in the neighborhood of $10,000 to program the app and submit it to the store.
That is to say people who checked "iPhone app" in the "how did you hear about us" section during signup. It was about 6-7 signups per month, and I believe it's still bringing in signups (I no longer work there).