So is $45 for an exhaust pipe. $700 for a plastic hood. $450 for a cheap flimsy grille. $400 for the clutch & steel pack for the PTO clutch. $100 for hood hinges..... I could go on and on. Virtually everything you will ever buy for these tractors is rediculouly over-priced. You could always bolt a couple of wooden planks to the framework and sit on that, but after a couple hours seat time, your arse will thank you for spending a bit of money for decent padded seat.
It's just the nature of these beasts.