Yes, an arthritic mare with 10+ years of life ahead of her is going to cost $$$$$$ at the vet to manage the arthritis. I would say $500 or less for a price; especially if you know she is going to a good home.
With her arthritis, her bloodlines really mean squat, at this point. You won't be able to compete heavily with her, and it would be dumb to breed a mare whose body is falling apart at such a young age.
But she may have a very happy life as a lesson horse, as long as she can be kept relatively pain-free.
∞ē*˚ Βгįťţαňγ ˚*ē∞
It is not enough to know how to ride; one must know how to fall.