Wild caught Zaire are the big money ones. You are paying for the collectors, shippers, and general care of bringing these things in from Africa.
Burundi are so cheap because they are so wide spread in the hobby, those can be bought at any Petco.
I bought Moba at 6-7" for $180 plus shipping, because I got a group deal. Single, they were $220 a piece. The only problem with my WC guys is that they are really skittish.
Oh yeah, like the other guy said. Big ones 10-14 inch WC can go up to 500 or more.