JunglePredator: I find http://www.indiegamebundles.com/ & https://www.cheapshark.com/ really help with that. CheapShark will show the lowest price the game has ever been on about 12 different digital game retailers & Indie Game Bundles will tell you if it's ever been in a bundle (though not always easy to find the price).
That said I also tend to look at the current price when I trade because it's like buying gas... "you have to give me gas for this price because that's what it was last week". Sorry... not how retail works generally... it's now %current price% & you missed out. However... in the end it usually comes down to how much I want the game & screw market/value. :-p Since most my games are doubles (my budget usually doesn't include the ability to stock pile extras... extras I get from buying bundles & sales such as have 3 copies of Splinter Cell Convict... 1 for pre-order sale, 2 from bundles) they don't hold the same value to me usually. (Example... I have a copy Convict left... I'd trade it for some from GOG that's not nearly the same value because I'd rather have some of GOG wishlist items alot more then the game is of value to me.... though I've kinda given up on trading for now. (Couldn't even trade for the $8 BG2:EE :-/ )

Gonen32: Afaik, Infinifactory (for example) isn't available now in any bundle and I offer it for an equivalent of less than its all time low price if anyone was going to buy it anyway.

Amadren: