Are they? What examples are you looking at? It used to e that a lower production run of 29ers might make them more expensive, but I would think that's less the case now than say 5 years ago, at least with hardtails. My hunch is that there are other reasons certain bikes you're looking at are more or less expensive. People focus on drivetrain, frames, forks, but what about brakes,wheels, even tires?