Having owned a few older Deere's, I've noticed the way their prices on parts begin to climb as the machine gets farther out of production. Parts on machines in current production are generally reasonable, but if you're buying for a machine that hasn't been made in 15 years (your 790 was build 2000 - 2007), the price goes up for each year out of production.
Makes sense, it's expensive to keep and move infrequently-used parts for old machine, for years on end. I'm just thrilled they still have the thing, when I need it.
That said, aftermarket is always an option. I went that direction with my last PTO clutch on the zero turn, since Deere recently doubled the price on that part.