The biggest aspect to all this is the user base. If 3.2% (thanks Graham Lee) of all apps are paid for, then you're likely to get significantly less for a WP7 app than an iPhone or Android one. Why? Simply because there are much more Android or iPhone users.
This link puts it into stark contrast: think that there's £3.5 million revenue available for every WP7 user in the world... Apple makes, $200 million per month!
The link doesn't give numbers for WP7, just Android and iPhone, but it looks right - Android users buy half as many apps than the iPhone users, but they do click on more ads. I've seen blogs from phone devs where they say that they make enough money from the ads on Android.
Oh yes, that link... from 2009. Imagine how much they get today.