As others have said, Mr. Pinball price guide, Boston Pinball and Pinside. I actually consult pinballprice.com occasionally as well, although information on many EM's or early SS games can be somewhat lacking or even non-existent on that site at times.
One of the problems I've encountered in this hobby is the lack of transparency from sellers on what a game actually sold for, or from buyers what a game was actually acquired for. I have always been transparent when buying or selling so people understand the price I'm asking, and I have no problem telling someone what price I bought a game for, or what I sold it for. There's absolutely nothing wrong with listing a game a little higher to give yourself some wiggle room with all the lowballers out in the world these days. If I list a game for sale for a slightly higher-than-normal price, I have no problem telling the potential buyer exactly what I paid for it, what I did to it while I owned it for improvements or refurbishing, and what money I put into it. I am always open to negotiation, assuming it is reasonable and I'm not getting lowballed. I have no idea why some people are so "secretive" about what they pay for games, or what they sold them for. Unless you are a retailer performing HER on every game, transparency would allow all of us to get games that we want or have been searching for, and for a very decent or at least reasonable price.
I will say, for the most part, several people I've dealt with on Pinside have given me relatively decent prices on their games. There's always the occasional exception. And I've met up with several Pinsiders and played their games, and they have no problem telling me what they bought them for. Awesome.
Unfortunately, there are many people in this hobby that feel they always have to turn a profit, and sometimes a very significant profit, on every single game. I often ask people what they paid for a game, and I get dodgy or evasive responses such as "I got it for a decent price", "It was an ok price" or they just dodge the question altogether and change the topic quickly as if they didn't hear my question.
Then there's flippers, which kind of ruin the hobby for everyone. I've often been looking for a specific title for quite some time, then I finally come across the game for a very good price. Unfortunately, someone who doesn't even really want the game grabs it before I can just to make a quick buck, and then you see it relisted within a week or two (or even faster) for some ridiculous price, now making it unavailable or unreasonably priced to all the hobbyists that had actually been searching for that game for a long time.
For example: A guy in NJ listed a Black Hole on CL last year for $400 or $500, don't remember exact price, but it was fantastic. It had been stored in a warehouse for years, so who knows what repairs it needed. But for that price, that left plenty of room to spend some money on the game for repairs without concern. I had been kicking myself in the ass for selling the first one I owned, which was cherry, so I desperately wanted another. I responded within 30 minutes, but he indicated the game was already spoken for. If someone that wanted a BH got it, I thought "Good for them. I missed out." But NO. I see the game listed on eBay the following day for $1800. Seller was not willing to budge on price when I contacted him, even though he admitted the game was the one from NJ that I had seen on CL. He had done no repairs to the game in the day he had it, just cleaned it up and wiped it down with Novus. Irritating, given that I could've gotten the game for a great price, and it would still be in my collection today. (Luckily I found an excellent BH a few months later, thanks to jkashani on here). But I was certainly not willing to give the seller more than 4x what he had gotten it for just the day before, especially knowing that it still needed repairs, and especially because $1800 is toward the high end price for a BH in excellent condition (for the non-retail secondary market). I wasn't about to give someone a $1300 profit for doing nothing, other than taking a drive. Had he restored it and then increased the price, I may have still bought it.
Sadly, there's a lot more people in this hobby now that will snatch up every cheap game they come across just to sell them at a marked up price, even when they did nothing to the game while in their possession. That prevents people that actually want the game finding one for a good price, and leaving room for repairs or restoration without being grossly upside down on the pin. I think that partially accounts for exorbitant pricing these days.
I don't blame anyone for trying to make some kind of profit...this can be an expensive hobby. Just leave the games you don't want alone, so someone that actually wants that particular title can get it for a good price and feel happy (and relieved).
Didn't mean to be so wordy or get off the OP's simple topic, but I had some thoughts on the subject.