Granted, they pay there developers a ton of money, but once the game is produced, they produce each disc for pennies. Not to mention map packs. They already make a killing off of games, there is no way in hell I'd pay $100 for a game. I have a hard time paying $60 for them as is!
Most games I wait until they're under $20 used, especially single player only. The value just isnt there unless its something incredible like portal 2. For cod, I usually go out of my way to pay $80 just to get it a few days early.
The value proposition is just ridiculous - a super polished product with a great 5 hour campaign without filler that's worth playing through at least twice, a solid coop component, and hundreds of hours of MP fun.
It's easy to say each disc is pennies but it's not like theyre cutting corners on the production of the game.