to me there is no such thing. "The amount the deal is completed on."
It relates to so much: condition, year, mods (if any), geographic area (of USA), type of area (city, rural), recent similar sales, from who, geo distance between sales/purchaser, ...ad infinitum.
$75 - $351 (if no mods)
Just ran a thread on this. There were several head reconditioning whole sellers. Seems ATK may B only 1 (nationally) left. Summit just stopped. Not sure if this ATK (at Jegs) is same co w/name change or new 1.
Used: I like car-parts.com. They have national presence for networked yards. Plug in Y/M/M, then narrow the geo scope if wishing.
I think the C9 is what we call here the "M" head (1969 i6, ie stang) & is good for the 'direct mount' carb (milling). A slight increase in price might B expected by some sellers (who know).
Sorry 2 B so inscrutable but that's how I C it. None being cast today (except the alu & its detachable intake) so go w/ur gut. Huston area should have plenty. Seek out the '77 + as it's more 'sporting'. Read above 'tech archive' (@ blue bx w/crossed screwdriver/wrench) & get the i6Performance Handbook (frm Matt @ vintage inlines dot com) if really wishing to know these engines!