What if MacBook will No Longer Detect Battery??

I am about to buy a MacBook off of a different forum, but the description says that the battery no longer takes a charge, but the seller usually just uses it on AC power. I know that this could just be a case of the battery at the end of its life (its a Core Duo BlackBook), but what if the connector has gone bad? Does anyone know how much the Genius Bar will charge me if the connector has gone bad and has to be replaced??

Edit: He says that the cycle count in About This Mac -> Power is 0. Is this definite that the connector is defective?

It means there is a good chance that the connector is defective. That means that it believes either that there is no battery attached, or that the battery has been fully charged and discharged 0 times. So, unless that battery has never been used, it means the connector is probably defective. As for the cost for replacing, you probably won't know until you ask them to do the repair. If they have to send out the book, then it will be at least $229 (if I remember correctly).

Edit: I don't know if it's 100% definitive though. It could also be a glitch with the counter for cycles or something else. Ask him to check what it says under "battery installed"