As far as I can tell they really aren't. From what I have read and heard, they seem to go for the $400-$500ish range. KLOV lists them in the common catagory, so that bodes well. Whether there is one a reasonable distance from me available, that is the question.
My good friend got a Simpsons pinball locally for about $600 and he restored it almost completely for around $300, so aa total of $900, which is very reasonable for a Simpsons pinball - well, good luck!