Not many around - I can get a brand new VHPD un machined no valve seats or guides are a lot of $$$ so I just found out.
What should I pay???
The answer is almost in your question… Not many around!! So its down to how badly you want said parts vs how much you can afford. Sorry not much help but supply and demand really
True 
Is that just the head or a full engine with crank, flywheel etc. The special bits in a vhpd are the head and associated bits, crank and Pistons (I think).
A complete brand new VHPD would be worth a lot to some people but it would probably take a long time to find the buyer. 6-8k? The target cars for the engine is small. Someone building a concours S1s Exige or Elise 190. I can’t imagine a Caterham owner being interested.
Just a bare head. I think I better get it as they are so hard to come by especially new