I my experience the price difference can be pretty severe. The biggest killer seems to be frames.
At one point found a Surly LHT frame on Craigslist for about half the new price. Did a build sheet pricing out the remaining parts and it was substantially more than a new, complete bike. Some of the stuff I already had. The only thing that made sense was to go high end. If you are going to pay more than msrp for a bike with stock parts might as well go all in and get the best of the best.