[H] Dungeon Defenders + DLC [W] Skyrim

but you can get dungeon defenders with all dlc for around 6$ on new humble bundle

But it is not what you paid for it that matters, it's what it is worth here and now.
Metro 2033 is a prime example of it. The game sells on steam for around $15. take into account every single major sale they have, it drops by 75%. add to that the amount of Keys for Metro 2033 that are flying about, makes the game almost worthless. it is so worthless I have seen it being traded for delve deeper (no offence intended to the creators of Delve Deeper)! However correct me if I am wrong the Giftable version of it would still be worth around $9 if it were not for all the keys?
Same as your Dungeon Defenders. it may be sold on steam for $49.99. but that would be a giftable copy. yours is a key, which are always worth less than their tradable doubles. add that you got it from a bundle, and the fact it is in the newest HB, the game is no longer worth as much as it was. Your key will only be worth another bundle key(it may push for Serious Sam 3), no matter if you paid $6 of $120 for the bundle. However if you were to keep the Key, in about 60 days you may get a game of the caliber of say a $15 - $20 game, but it depends on how many people missed the bundle, or didn't get one in a trade. If you had a gift copy of the game, you might be able to get Skyrim in about 60 days, because it will be worth more than a key.
EDIT - you also need to take into account how many times the game has been in a bundle, or how often it is in a bundle. the price will continue to dropp if it is a regular.