Don't forget, Marco is not shy to charge $4.99 for Instapaper in the App Store. I do not have the statistics right now to back it up (although I'm sure a few Google queries could), but I'm fairly confident that's above the average price for an app.
The iPhone app also has some value to it that makes it worth the $4.99, e.g., 250 article capacity, iPad compatible, and it remembers your position.
The only benefit subscribing gives to you right now is being able to hide ads, which people can already do for free, and the warm, fuzzy feeling you get from supporting Instapaper's development.
Not to be nitpicky, but there are some insanely expensive apps (medical, etc.) in the store, but probably most are under $1 — so for distributions like that, the median is much more useful a statistic than mean.
