No game is worth $20 if you ask me....
With that said...I do have Halo 4 on preorder (but this is my first game I have preordered since Halo 2)
...just check ebay. Seriously I do this all the time!! I never pay more than 15 bucks for a game. Once it gets 6 months old...you can usually start to see them really cheap on ebay or half.com.
There is a Buy it now priced at $15. Took me 3 seconds to find it...im sure if you spent a day or two you can find one for like 10 bucks in the auctions