The current version of Maps Engine, which is largely aimed at developers, has usage-based charging. However, Maps Engine Pro (itself built as a Maps Engine app, with an Android client also available) charges on a per-user basis. You can pay $US5 per month per user on a no-contract deal, or $50 per user per year on a 12-month contract. (You can add additional users to the 12-month contract on a pro-rata basis).
You can sign up for a free trial before using the full service. See Google’s blog post for more details.
Introducing Google Maps Engine Pro [Google Enterprise Blog]