You don't have to pay to use google maps API on mobile devices.
From this post:
The official answer from Google:
http://code.google.com/apis/maps/faq.html#usagelimits
The Maps API usage limits apply only to the following APIs:
-Google Maps JavaScript API v2
-Google Maps JavaScript API v3
-Google Maps API for Flash
-Google Maps Image APIs
The Google Earth API, and native Maps APIs for mobile platforms such as Android and iOS are not affected. Use of the embed feature of Google Maps, and other Google products that offer an embed feature that includes a map, are not affected by these limits.
EDIT
The terms have changed since the question was first answered and now mobile API's are subjected to the same limit as web sites: 25000 map loads per day.
