Cool!
Idea: What if every page load, you displayed an app prominently, weighted by the cost paid for that tile? For example, if you sold 3 tiles for $1, $2, $3 respectively, with 1/6 probability you display a prominent ad for the first, 2/6 a prominent add for the second, 3/6 a prominent ad for the third (maybe not a linear scale but you get the idea).