In any case is not that they're planning a XIII-4, it's obviously going to be the last unless they decide for a spin-off on handled later, but that will cost even less (a la Revenant Wings) so it'll not be important how much less it'll sell.
But expect to see this model applied to the future of every Final Fantasy, it's way more profitable to them than continuing other spin-offs series like Crystal Chronicles and Tactics. Fans should also be more satisfied actually.
How much less it sells actually matters, because that determines how much more time and resources they're willing to sink into that particular IP. If they budget less and less for subsequent games, you're not going to get the quality you want or even another game.
Think about it this way: if 13-2 sold 25% of what 13 sold, then to make the same ratio of expenditure to profit they did off 13, they had to have spent only 25% of what they spent on 13. Even then, they sold only 25% of 13; that's not increased profit overall, but much decreased.
They should be making sequels with the expectation that their audience would have expanded from the previous game rather than with just the hope that the sequel will recoup more of the investment in the first game. Franchises with sequels this gen, like Uncharted, Gears, Asscreed, Elder Scrolls, Mass Effecttheir sequels earned
more, not less.
Yeah probably XIII-3 if it exists in the intended form will indeed sell as much as XIII-2 if not just a bit less.
The pattern thus far for the XIII series is that the sales of the previous game is a ceiling, not a floor. XIII-3 will most likely sell less.