DON'T buy some price guide. A waste of time.

Go here for more accurate pricing: http://www.bostonpinball.biz/ebay112315.htm

Was just updated in November.

A few titles are off but overall, it's the best free pinball pricing resource that you are going to find.

Sure there is...
highest price=best condition (like new or new)
lowest price=worst condition (parts machine)

I would argue that it's better then speculative condition, in a way. Also, if good pics are provided for a given sold game on eBay, I trust THAT over what some "guide" speculates is the current going price.

After all is said and done, completed eBay sales is just a "guide"
A yearly price guide is also just a "guide"
Your personal experience can also be a "guide"
Pinside forums/posts/asking prices could be considered a "guide"
Local groups to you, shows, leagues, ect. can also be a "guide"

All these in combination will give you the best information.

I will admit that the worst thing about the Boston list is that the more rare a game is to locate, the less likely the guide will help you in determining the potential value/asking price.

But you left out the MOST important variable: location.