Notebook batteries aren't cheap these days, and our own Conrad Quilty-Harper from across the pond (of both TUAW and Engadget fame) blogged a little experiment to give his MacBook some new mobile legs. With a total of 11 parts replaced in six months (which is far more than required to put a machine to sleep for good), he decided it was time Apple threw him a bone and called support for a battery replacement. The Apple rep told him that if he didn't return his old 'n busted battery upon receiving the shiny new one, they would charge him £71 (~$138 USD). Fair enough, but a trip to the UK Apple Store online revealed that new batteries off the shelf cost £99 (~$192) - and that's quite a difference in price no matter where you hang your hat.
Now we aren't sure if the math works the same way in other countries or even the US, and we don't exactly condone calling Apple for no good reason to score or even swap out for a new battery, especially since Apple could very well charge you anyway for making a bogus call and sending them a perfectly functioning battery (i.e. - pull this stunt at your own risk). This might be useful, however, if you're truly experiencing power issues whilst untethered and saving every penny on a replacement counts.