Dan,

Your best bet is to bring it to a decent shop and have it looked at. Decide if the repair out-weighs the value of the bike, might be a quick fix, who knows though, take it shop and they may have a suitable replacement part on hand. I will be decidedly difficult to help you over a forum with what is basically a department store bike, not disrespect to your purchase; it's nice that it's lasted this long for you. If the part is available my guess is you would expect to pay about 15 bucks for the part and $40-50 in labor +/- $20. If the BB is in that kind of shape I'd be willing to bet that i needs a multitude of consumables replaced.