They are pretty much the same price as any other new dirt bike. And it usually costs more time and R&D to build something light and strong. And considering the production numbers are a fraction of a regular dirt bike, I'd say they are pretty reasonable.
Trials bikes also hold their resale value better, since it is a limited market and the cost of ownership is quite a bit less than regular dirt bike too, so trials bikes are less to own, if you factor in all the expenses. Plus you can buy a new GG Economy model, essentially a two year old Pro for $5500.