I can answer the first part of your question.
ALL A series heads will physically bolt onto ALL A series blocks, but there a couple of points to look out for.

Front distributor engines have oil passaged drilled in the block & head with an 'O' ring to seal where they connect

Rear distributor engines had revised & simplified rocker oiling drillings.

Early heads [front distributor] can be used on ALL blocks without problems

Late heads [rear dustributor] can NOT be used on early blocks unless a suitable mod is carried out to get oil from the block into the head.

Later heads from A14's & A15's used larger valve sizes than A12 engines, so just make absolutely sure that the edge of the larger valve does not clip the edge of the smaller bore in an A12 engine. The factory machined the bores for valve clearance with the smaller valves, so in some cases, it might get a little bit close.

As for average value, ..... beats me.