I bought my iPhone in Feb 2009. I saved all emails from iTunes and got bored and wrote a script to extract the information on my purchases. Here is a breakdown
I've downloaded 316 apps (I currently have 21 apps on my phone. I once had 9 pages of apps, but realized I never used them, now I only keep what I use often.)
I've spent $25. I'm not cheap or broke, I just don't buy things I know I don't need. I'm also not into games and have only bought a couple.
Out of the apps I paid for, 2 were actually mistakes. One was a game which I was going to try but accidentally got the full version, and another was a book I thought was free.
I've only continued to use 4 apps regularly after purchase. Two I could live without. Two, Prowl and RSSPlayer, I couldn't live without and they were also the most expensive at $2.99.
iPhone and Linux