The price will be $0.0005/URL. Pay only for what you use, there is no minimum fee.
To get you started, the first 500 urls will be free (note: total, not monthly).
The request is charged even if the results have been cached by us. This is because (1) there is always a charge for us, even if it's a little smaller than normally. And (2) because you should actually be doing your own caching — this is more economical for all of us.