BBM uses data, so the only charge would be for the data it uses. There are not texting charges. (Data usage should be minimal unless you are exchanging files, or belong to active groups)
It depends on the carrier. Most U.S. carriers don't charge extra to add BIS (BlackBerry Internet Service) to a data plan beyond the data plan cost itself - whether it's one of the tiered plans or unlimited.
Outside the U.S., however, is a different story. I know many non-US carriers charge extra to add BIS. Sometimes a little, sometimes a lot.
Posted from my CrackBerry at wapforums.crackberry.com
Well, without your providing us any context, it sounds like you're referring to an unlimited data plan. But as no one in this thread has mentioned that phrase, I can only assume.
Once you have a data plan, then your BBM should work just fine.
I'm on Virgin Mobile USA's unlimited data/text/email and 1200 min plan for $40/mo and then $10 for the BIS plan. The BIS includes the BBM, Blackberry App World and browsing...