That's a difficult question. What's a good set ? Do you have the rest of the parts ? Do you mind the spare part case number ? Does it have a (matched) timing cover ?
Reasonable complete engines seem to be around £250 and should have plenty of usable parts.
If they're really NOS and have absolutely no damage or wear on the lugs (the biggest problem on used cases) then I'd guess £100 - £150 because they would form the basis of a good sound motor. If they're a bit scruffy and need any welding work then £50 - £75....but it's guesswork on my part.
You can still find knocked about cases for £25 at UK jumbles