Pricing varies with game, condition and location. An accurate price guide is impossible to achieve.
I used to buy the mr pinball price guides all the time when i first got into the hobby.
After 13 years owning, buying and selling, there are few games that i do not have a ballpark price in my head of what they are worth.
I have only owned about 35 or 40 games but i watch for sale ads all the time.
While i have only sold one game on pinside i must be close in my estimates because i never have trouble selling my games locally.
I think one big issue is wanting to profit on a game. For me, it is a hobby. More than once i have been upside down. The one game i did sell on pinside, i sold for 35% of what i put into it. Thats life. I am not into it to make a profit. I enjoy fixing and restoring machines.
If i want a particular machine, i know what the range is based on condition and buy accordingly. I havent looked at a price guide in years.