Effective May 5/2009 starting with ABPro version 1.4.2 beta 2.

GNU/GPL - Open Source

http://www.gnu.org/licenses/gpl-2.0.html

You can use it on any site(s) you wish. 

You are under no obligation to pay for additional domains.

That said, you expect your client to pay for your work.

Perhaps they could help pay for mine.