I always refer to the Boston Pinball site:
http://www.bostonpinball.biz/

click on the "eBay Sales Summary" button.
(I'd give a direct link, but it's updated now and then so best to get there from the home page)

It only shows eBay sales so it's not comprehensive. But it's a pretty decent gage for comparison. Seems to do fairly well on market values. Note these are US prices too, but still useful for getting a good ballpark idea of values, especially comparing one pin to another.

edit to add: I got to thinking it's just eBay, so it would include US and everywhere else too, so maybe more accurate than I previously considered.