If you implement optimizations, you can really significantly curb the cost.
I spent some time tuning SharedCount's API, which would have cost me $30-$50/day, and its now at about $1-$2/day.
- Move to Python 2.7 and enable multithreading
- Setup Cloudflare (this swallows about half of all my requests)
- Increase minimum latency and reduce the maximum number of idle instances. (I have 5-8s and 1-2 set, respectively)
- Setup the semi-undocumented Google edge cache (basically, just a Cache-Control: public, max-age=[seconds] header.
- Take advantage of memcache.
With this setup, I'm doing 3 million API calls per day at $2.