70's GM Performance Heads

[Thumper71]

I currently have a set of GM "Turbo" heads which were classified by GM as service replacement heads, but really they were the first part GM offered back in the 70's for high performance applications. They are cast iron, 64 CC, 2.02/1.94, 180 runners. I would like to sell them to put that money towards getting set of AFR's. They are used, but still motoring me around. Does $500 for the pair seem reasonable? From what I gather they are fairly rare to find as most of the racers back in the 70's snatched these suckers up.