Apple Thoughts has an interesting discussion about how the average iPhone user downloads 9 games.
Are you an average iPhone user? Above average? ...below average?
I probably have more than 9 games. All were free. Mostly, I put the games on for my kids, but I don't let them play with my iPhone much anyway. They do all kinds of kooky things to my wife's iPhone. I let them play with my iPod Touch, with the wireless disabled. Somehow they deleted one of their favorite games. Occasionally I'll get a call from my wife to find only background noise, leading me to assume that the kids butt-dialed me.
The story Apple Thoughts links to says that games are the cheapest apps, most of which are free.