Development studio Bethesda Games is hard at work on Fallout 4. Naturally, they haven't concocted any of the DLC the game is expected to have yet. They'll think of that stuff when they have time to think again.
Whatever it decides to put out, Bethesda DOES know THIS: the Season Pass to purchase all this DLC at once will be $30, no more. The first round of this DLC is expected sometime in 2016, and there will also be minor updates before then that will be free of charge.
The $30 price will cover "all of the Fallout 4 DLC we ever do," the developer promised. "Since we're still hard at work on the game, we don't know what the actual DLC will be yet, but it will start coming early next year. Based on what we did for Oblivion, Fallout 3, and Skyrim, we know that it will be worth at least $40, and if we do more, you'll get it all with the Season Pass."
Other software giants tend to charge $40 or $50 for their Season Passes, and this is when they DO know the actual value of what they're selling. Bethesda's being incredibly nice to you guys. Now where is their thank-you?