I think a "fair " price is subjective. For a hard to find piece that is desperately needed any price is is a fair price if lack thereof can stall a project forever. In your case, a fair price would be the same as what it would cost you to have the piece fabricated, assuming the part is in as good a condition and is as useable as the piece you have to have made.
When I got my 56 TBird, it was missing the factory jack. While I could have done without it, I wanted it to have the full equipment the car originally came with. I was willing to pay what was asked because they were not easily obtainable
That is my take. Others may differ