I released a GAE app which does something similar.  However it differs in that it is automated and it emails you a daily digest of your sales, upgrades and current rankings.  In the email are also graphs of your sale and upgrade history.  I've been using it successfully for several months now.
It's also written in Python and on GitHub.  :)
<a href="http://github.com/baz/app-sales-machine" rel="nofollow">http://github.com/baz/app-sales-machine</a>