I will ask $500 or best offer for my iron gt40's soon, when I take them off. Slight milling, peformance valve job, and Comp Cams springs. Going to AFR's soon, I hope. Seems like a fair price, they should only have about 500 street miles and 50 runs on them when they get ready to come off.
Be careful. If they are ready to run, they are worth more than if you have to work on them. I spent a bit over $850 on mine, including buying them, adding the spring setup, and machining.