It doesn't, apparently. The cost seems to be per-user, so
250MB asset storage / user
3MB database storage / user
50MB transfer / user
10 req/s per 100k users
is all free. I don't know how the requests part scales though, 10/s for 100k users is each user hitting it once per 3 hours. I can't complain about the amount given away for free, but I don't know quite how this would scale with costs for different types of apps/loads.
