How much does a Post Office box cost?

Depends on what size is required. Check out this website:

http://www.usps.com/receive/businesssolutions/poboxservice.htm

My Post Office has three sizes of boxes: small, medium, and large. The rate for the medium, which is needed if you receive a lot of letters or packages and/or do not pick up your mail on a daily basis, is around $55.00 every six months.