Part of it is the cost (which is worth it) but also the fitment as it makes a big difference vs. a seat made for a different sized rider.
I've had 2 used Russell seats. One was made for a rider within an inch of my height and within 5 lbs of my weight. I don't know what the other one was built to, but it did not fit me right and was not comfortable.
The one I kept is perfect. The only thing wrong with it is that I didn't get it sooner!