Love my Simpsons DE! My first grail pin (yes, there have been more grail pins added to my list after it).
I reckon around $3500 max for a NOS playfield and completely shopped out machine sold to a Simpsons fan? It's a popular themed machine, but it seems lately that demand may be slipping for this title and average ones seem to sit on eBay. When I was looking for one it seemed to be a rarity.
I'm basing my price on another popular themed Data East, Star Wars. Good ones seem to be getting around $3400 on eBay of late.
As MartyJ said, the current price of that machine may be maxed out for people in pinball circles, but I think there still might be that Simpsons collector out there who will pay for something nice. Depends how quickly you want to sell it?
Cheers!
Adam.