I could definitely see P4A being a PS+ game at some point in the future. Its retail price has absolutely TANKED (I've seen it for as low as $15 at times).
It really hasn't tanked that hard given how "old" it is. It's shown up a couple times on the gamefly sale for $15, but it sold out within minutes. I just bought it last week and couldn't find a better price than $25-30. For an almost year old fighting game, that's not bad at all, though I'm sure the supply is much lower than something like Street Fighter.
Game is great though!