It's written in Python. Hosting costs about $700/month on Google App Engine. It doesn't cover our costs yet, but it's growing. Hardest challenge so far has been to find ways to let the world know about it.
App Engine has it's disadvantages as well. At one point I used typhoonae (open source framework that let's you run you GAE apps on EC2) and was about to switch, and then decided against it.
I expect the cost per user to go down as the percent of paying to trial users goes up. Trial users stay for 30 days, and then most of them stop. But paying users continue using it much longer. Also, there is a lot of room for optimization, which I'll get around to doing later, once the key features are in place.
- 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.
Also storing objects that don't need indexes as pickled blobs in a parent model helps a lot. The advantage over JSON is that you don't need extra code to parse, (de)serialized and validate the object.