“As stated in numerous Activision Blizzard quarterly conference calls, Modern Warfare III is a premium release,” explained an Activision spokesperson. “It will be price accordingly at $70 USD.”
I think everyone should chill out and wait till Thursday, see the trailer and all of the news about the game, before going crazy about if its an "expansion" or "full game" or whatever. We haven't even see all the details on the game yet! Form an opinion after you do!
While the game is listed as an add-on on Steam, CharlieIntel explained in their Twitter thread that it’s due to Call of Duty HQ, the publisher’s app that’ll be used to streamline and house all of the series’ releases going forward. So while “add-on” might suggest it’s a truncated release, it’s not the case according to Activision.