Wow, that's rough. I noticed on my brief foray into AppEngine that it is really easy to run up against their CPU limitations (as in, four trivial queries on a single pageload will do it), but it's amazing that you've managed to run up a $1,000/mo bill with them.
Or are you really burning through SQS requests fast enough to make it cost cash money?
Either way, it sounds like you've painted yourself into a corner it will be hard to get out of. AppEngine is probably the most difficult platform imaginable in terms of how hard it is to migrate off of. You'll have a ton of work to do just to get out of BigTable, and then you're left with a Django app.
I guess the best you can do is start charging money, and start developing a new version as quickly as possible to stop the bleeding.
Good luck!