You should figure out what a game is worth to you first and then see if you can find it within that price range. The best way to do that is to play through a game before deciding whether to buy it..
There aren't any accurate resources for pricing, but there are a lot of misleading price and rarity guides out there. The only way to get an accurate feel for what any given item is selling overall, you have to keep an eye on selling and failing-to-sell prices for several months. It's not a big deal if your aren't after an insta-set. Just start off buying games you feel comfortable about the prices for and buy and play them as you watch out for the rest, particularly waiting the longest to get a feel for prices you are most unsure of.