I personally find Boston Pinball to be way too low almost always. It must be skewed by projects or old info. Pinside prices used to be way too low also, but whatever changed recently I think the ranges are pretty good now. Pinside also doesn't try to make up a price if there isn't a lot of data, it will say "Unknown".

Mr Pinball's price guide gives you a decent ballpark if you want something in print, although for rare games and flavor of the week games it sometimes misses rocket ship. You'll see Mystery Castle for $1000 or Congo for $2000 which are pretty far off. It also has symbols to indicate if a game has gone up/down or way up/down from last year. Kind of fun to browse old price guides and see all the games I would've been able to afford in 2002.

Prices vary in range and are too volatile to really get better than "decent ballpark" for any price guide.