Sorry, the only thing that has been consistant with the FF releases after FFVII is the fact that a mass amount of the sales have happened in a months period or a short amount of time.
There's a reason that the FFXIII group is aiming at the nostolgia of the older games. Clearly the sales of FF games to date shows a scale of ups and downs, technically the FFXIII release should be an up because that's the pattern the sales are taking, but they won't risk that, so there's 3 FFXIII's and their aimed at old AND new players.
Here are the sales to date.
FFXII - 3.9 million
FFX - 6.6 million
FFIX - 4.6 mil
FFVIII - 6 million
FFVII - 9.5 million
FFVI - 2.55 million in japan, 550k in US for SNES,then 364k for anthology, not including GBA
FFV - 2.45 milion units on Super Famicom + 364k on the Anthology release. Not including GBA
FFIV - Almost 3 million copies including original and PSX release. Not including GBA
FFIII - 442500 units
FFII - Couldn't find stats
FFI - Couldn't find stats.