I paid $1800 for my set which freshly rebuilt, free of cracks or damage, extensively ported and fitted with 2.35" / 1.73" (tunnel port) valves 4 years ago. I thought I got a really good deal considering the power they make and that they were correct for my car.
I'd guess here in N. California a stock good running (fairly fresh) set free of standard Medium Riser heads free of cracks or repairs would be worth $1200-1500. I've seen freshly rebuilt C5AE heads sell for as much as $1800. C5AE heads are by far Fords best all around head. They flow nearly as well as the Tunnel Port heads at 296 cfm v. 308 for the TP head. They actually flow a little better than the TP heads at 177 cfm v. 166 on the exhaust side. Potential power for the Medium Rise heads was 609 v. 634 for the TP without as much low to mid range loss in horsepower.
Here are a set of 427 C4AE heads (not medium rise) for sale now. They are fresh and can be had for $650. Track them and see if they sell. I can't find a comparable 427 head for sale right now. Nothing listed at Carls, nothing at DSC.
http://cgi.ebay.com/ebaymotors/FE-42...QQcmdZViewItem
I started wondering why I couldn't find a set anywhere and on a lark I called Carl's Ford Parts. Carl told me they had nothing for sale but a "decent set" would be $1800-2000. $%^&*() Wow. I'd keep them for awhile and see where it goes. You won't be the only one.