The FileMaker Go iOS App has been free from the Apple AppStore for some time.
When hosting databases using FileMaker Server version 12 there is no additional cost for FileMaker Go users. Hence after purchasing a FileMaker Server license and FileMaker Pro licenses, adding iPad/iPhone users did not attract additional license fees.
This changes under FileMaker Server v13. There is now a cost for allowing FileMaker Go users to access databases hosted on the v13 server. A certain number of 'concurrent user' licenses must be purchased to cover the maximum number of simultaneous users.
Find details of v13 pricing here.
Find and explanation of Concurrent Users here.