Currently the API is priced at $0.01 per call, with $10 of free credit when you signup.
We'll likely add monthly subscriptions in the future perhaps with a recurring monthly free tier, but we don't have them yet.
If you do run out of the free $10 credit and want more time to see if the API is going to be useful for you shoot me an email and I'll be happy to issue more credit.
UPDATE: The free tier is static at 1k and then you have to buy credits or it is 1k / month?