A pinball is always worth the price a buyer is willing to pay.

EMs like Gav's $5 pinball is one end of the spectrum, and the Mermaid that sold in the US for $20,000 is the other.

KISS is a classic. As a pinball, it is worth what all SS from this period is worth to a casual buyer - $1500. But a KISS fan will pay $10,000 for a restored game.

There are so many factors to take into account. There's "market value", "the last eBay sale value", "Dealer's value", "Collector's value" and "casual buyer's value".

I always look at the "games in person" before deciding it's worth. At least detailed pictures and a report - "working" / "not working" etc etc..