Whatever someone is willing to pay. I wouldn't think the demand for that would be very high, though. I think it would be difficult to move that one at any price.
If the "FB" is used to mean "Facebook", then you will find yourself involved with a trademark lawsuit -- which you would lose. If "FB" means something else, I have no idea what that would be. If you're not sure what "FB" means, then the domain is worthless.