An analysis of iTunes revenue by Asymco suggests that the average iTunes user spends $US16 a year on apps. With the vast majority of app downloads being free, developers have to work hard to persuade consumers to part with even 99 cents.
Asymco's analysis suggests the average user spends $US16 a year on apps, $US9 a year on software, $US12 a year on music and $US4 a year on video. As an average, those figures are undoubtedly flattened by inactive iTunes account holders, but even if you double them it's still not a massive amount of revenue to chase.
We've seen via analysis of Apple's own official (if scant) numbers that the vast majority of app downloads are for free apps, and this data chimes with that trend. The $US9 a year for software (which means Mac software in this context) might suggest another route, but it's worth remembering that number includes paid-for updates from Apple itself (such as major OS X releases) as well.