I think you have hit on it Barry, the condition of the bores and the valve seats will deturman the price. Some purchased by a friend lately for $500 a set, turned up to have .080 overbore, and chimney style bores. Small at the top and big at the bottom. The were very nice looking, but that doesn't help when you need to run them. Find out more information, and then ask. Ones in good shape are very desirable. I wish mine were still at .030 but mistakes have taken them to .050. I may have them sleaved before they get too thin.
Very much worth persuing though!