It's not just buying the bearing because you then need someone with a press to actually press the old one out and the new one in. Actually by the time you need to replace them you can usually just beat the old one out with a hammer yourself.
So if you buy one from rockauto.com or whatever, you either need to buy the bearing and then get it pressed in at pep boys (usually $30) or buy the entire hub assembly. If you have ABS, buying the hub assembly is spendy because of the ABS sensor. If you can find a hub without the included sensor, you're going to do much better.
