I was able to get my GAE costs way down by focusing on minimizing DB writes. Some tips:

- Instead of storing an array in the database directly, store it as a json string. storing an array uses a DB write for every array element.

- Don't index things if you can get away with it (indexing doubles writes).

In my case I was using about 600 writes for every user action when I only needed two. This may be common knowledge, but I was pretty surprised that storing arrays was costing me so much.