AAA is to do with budget/production value. it doesn't matter if we don't know the budget because there's no exact criteria. There is no fixed amount that a game must have cost to be AAA. A game doesn't have to be "good" to be AAA.
"Production value" includes not just the content but how ambitious it is.
AAA is a quick term we can use to differentiate games, even ones in the same series, such as xbl arcade/PSN titles from full releases
Assassin's Creed Unity is AAA, Assassin's Creed Chronicles is not.
Battlefield Bad Company is AAA, Battlefield 1943 is not.
Halo 3 is AAA, Halo Spartan Assault is not.
Rise of the Tomb Raider is AAA, Lara Craft Temple of Osiris is not.