I have ~100 visits per day. That costs me ~$0.10 per day.
More details here:
https://github.com/entaroadun/hnpickup/wiki/Hacker-News-Pick...
3.8 million data store reads for 8,000 visitors seems high. Especially for the kind of data you are serving - it's not rapidly changing.
Are you using memcache at all? A quick scan of the source doesn't show any memcache imports, but I just might've missed it. (http://code.google.com/appengine/docs/python/memcache/usingm...)
Probably the production version of the code will get memcache and some other features. Apart of couple suggestions from the discussion here the educational version of hnpickup will be frozen. Although, any simplifications or correction to the code or documentation are very welcome.
Denial Reason: Insufficient Text.