According to BBC:
From 1 January 2012, Google will charge for the Google Maps API service when more than the limit of 25,000 map “hits” are made in a day.
Websites, especially travel firms, use Google Maps to link customers to a view of the destinations they inquire about.
Google is rumoured to be charging $4 per 1,000 views in excess of the limit.
Google maintains the high limit of 25,000 free hits before charging “will only affect 0.35% of users”.

‘Secure future’

Google said it was aware that developers needed time to evaluate their usage, determine if they were affected and then take action as appropriate.
“We understand that the introduction of these limits may be concerning,” said Thor Mitchell, product manager of the Maps API at Google.
“However, with the continued growth in adoption of the Maps API, we need to secure its long-term future by ensuring that even when used by the highest-volume for-profit sites, the service remains viable. “