I have seen them anywhere from $3,800 to $6,000 depending on condition, but they do not come around that often. 2008 book value on it is $4,825.
I have seen several on Mr. Pinball Classifieds lately.
Good Luck on this one too... I have a Monster Bash as well.... We have similar tastes...
If you are ever up in the Weeki Wachee area, stop by and play...
Thomas