I looked at ebay under gembone dinosaur bone. There were several hundred listings and only 2 that I would consider gem bone. If you want decent quality agatized dino bone, there is a lot of decent bone available and at fairly reasonable prices with the better red and yellow colors having the higher prices. Unless you find an estate sale with some good bone, the prices I see on most of the pieces on EBAY are decent if the material is solid.
I personally will not buy bone without handling the material as I am concerned with cracks, pits, soft spots and weak matrix between the cells. Mark Anderson gave a talk at the Agate Expo on Gem Bone. He is known for his inlayed rings and other jewelry with bone. I have not yet watched the DVD with his presentation. He had a variety of bone including bone replaced by pyrite.
I have attached a picture of a piece of Gembone from Mark's website Different Seasons Jewelry
I wish you luck on your hunt.
Bob Johannes
The Amethyst Rose