Really depends on what we're talking about. I've seen used heads, complete w/cams from $150-$350 depending on mileage/condition. I've been talking with a guy in another forum who's got a freshly P&P set with stock cams and he's asking $550 for the pair. Intake also factors in. Shipping too!
I realize a high mileage set for say, $225 needing a valve job, etc., isn't any cheaper really than a clean low-mileage set for say $350.
Sorry if that's vague, but it's the most honest way I can answer.