This may technically be a "deal" since it's not MSRP, but actually this game has been at this price point on Amazon for years now... Same with FF8, FF Anthology, and FF Chronicles (all PS1, all Greatest Hits versions).
The only PS1 FF to retain any sort of value was FF7. And maybe black label (non-GH) versions of some of the others.
Shockingly, those games you just listed are no longer for swale from Amazon first party. They must have gone out of stock recently. I guess now that they can sell them digitally, Squeenix is finally turning off the "physical copy" firehose.
Though it looks like physicaly copies of things are still available in their online store for $15 each... Chrono Cross, Final Fantasy Anthology, and Chronicles, etc... it looks like VIII and IX are gone, they used to be there.