Compared to what?
I just got a new 2012 Sherco and found it priced in line with any other recretional offroad toy. I have co-workers who buy "normal" off-road bikes, quads, etc. and I have watched the values of those toys fall through the floor. Trials bikes do hold there value better (probably because the used market isn't flooded with 2-year old impulse buys gone bad). In fact the used market is strong enough that it helped justify just getting a new bike intead of a used one. The deprecession hit on trials bikes is much less then just about anything else out there.
As others pointed out, low production runs and nearly yearly updates will push up the R&D price that must be rolled into each bike. So while you think they are simplier, the piece cost of the parts will be higher. Ever price replacment parts? Notably higher then the mass produced "normal" toys. Paying for that lack of volume. But you are saving on not paying for an over-inflated dealer network and advertising.