i dont know a whole lot about it but i think some of them cost more because they are captive bred, raising any cham takes a lot of time, money and patience though..
also im guessing supply and demand has a part in it, they are the most popular that last i checked.mmm... a lot of reputable breeders seem to pick out nicer/brighter chams to breed and stuff like that.. kind of like buying a purebred dog or something.
i could be all wrong but thats kind of what i assumed when i decided to take the plunge