There are a few variables with the pricing of a WH2O machine mainly being topper or no topper.  For a nice WH2O machine you would expect to pay anywhere from $2500 up to the low to mid $3k's for a really really nice example with a good topper. 

Other price variables would include whether the machine has new ramps, new boulders, playfield wear near the Lost Mine kickout, re-decalled cabinet, LEDs etc etc.