Ever since Forza 4 they have being pulling that one. I don't agree with it and think the season passes are terrible. What's the point if you don't get all the content? Only bad point about Forza apart from micro-transactions.
Hopefully it will be reduced in price as the Porsche and NASCAR campaigns look pretty cool.
I bought the Porsche expansion when it was half off for deals with gold but there are quite a few dlcs that I'm lacking. I agree with you though the season pass is a bit of a rip off.