The free limit is 15,000/day/IP - if your users use your key using the GMaps javascript library, I don't believe that counts against your server's quota, since

The free limit is 15,000/day/IP - if your users use your key using the GMaps javascript library, I don't believe that counts against your server's quota, since it's coming from a distinct IP per request. 15k/day is a lot of installers... Edit - And my reply wasn't meant to question you, you just illuminated the issue well.