Seems like 2K each for all but the ecru citrine, is about average. Before they lowered the enhance potions bonus on the rings, people were paying 20K and more each for the ecru citrines, but I haven't seen anyone offering that since the change. There is still demand for the rings even with them lowered to 50% enhance potions though, so I would guess they still go for a lot more than the other gems.
-Skylark