You can buy it if you want, but be warned, only Square knows what it will actually contain.
Most of the time, when a season pass is announced, the developer will detail the DLC for their upcoming release, just so the public knows what they are purchasing. This is not one of those cases, as the PlayStation Store listing for Final Fantasy XV never details what will be included in the season pass. All it says is, ""The Season Pass allows you to download additional content.".
The season pass will be included in the Premium edition of the game, which costs $85. If you pre-ordered the standard version, and wish to upgrade to the Premium edition, you can do so, but at an unknown cost. One would figure $25 dollars, but it's never stated (as of this writing).
My Two Cents
Whether they want to keep DLC a secret or not, this is just shady business practice. 25 extra dollars is a lot of money to spend on essentially nothing, and Square doesn't seem to keen on revealing what the season pass is anytime soon. The saddest part? People will still buy the Premium edition, or upgrade from the standard edition, proving to developers that we will basically buy anything.