I love watching the Barrett-Jackson collector auto auction on television. One of the observations that the commentators frequently make is that most of those beautiful hot rods sell for far less than what it would cost you to build it yourself. The same is generally true of upgrading and modifying bicycles.
The flip side of the coin is that, for people like me, redoing bikes is fun in itself and you get a bike that's set up exactly the way you want it. To me, that's worth the extra money.