When a game has sold a certain number of copies, it is certified as 'Platinum', and the price drops. This doesn't necessarily mean there are any extras included, such as add on games or levels. However, sometimes when a game goes Platinum that has had extra add ons on another platform (such as a PC version), these add ons are included. For example, The Elder Scrolls IV: Oblivion. When I got the Platinum version of this game, it included add ons that would previously have been bought separately. My advice would be to look on various sites before you buy, different sites offer different versions. Have fun! :)
That's right, when a game has sold a certain number of copies then it should be certified as 'platinum' and the price should drop. Unless ofcourse the game is called 'cod4' in which case despite massive sales outstripping most games by a significant order, they are entitled to maintain their high prices and even increase them whenever they feel like it. Hope that clears any confusion.