Here's a related web app I would pay for:
1. Provide a way to add "apps of interest"
2. RSS feed with search over new apps with given keywords
3. RSS feed with updates to all tracked apps (from step #1).
4. RSS feed for new reviews for all tracked apps.
5. Dashbard view with historical ranks of all tracked apps, with
updates added to the charts as important points
6. Attempts to guess unit count of the tracked apps by analyzing stats
for other, known apps and rank.
7. Alexa&co ranks for apps respective web sites.
In other words, I would like ot have a "competition dashboard". I
would pay $20/month for it.
I already have a script that downloads daily sales from iTunes and
plots them on a graph, but I imagine many people don't and would pay
for that as well.