Hi Steve.Hi Ken,
The effort today to make a reproduction GTA bonnet, cost of materials and specialist labour and tools say (not using the original alloy) so reproductions are on the market for ~$3.2k USD.
Finding NOS GTA parts may be possible so a valuation of NOS GTA bonnet may be possible along with a used GTA bonnet.
I would say triple and upwards the number ~$3.2k USD for a NOS GTA Bonnet so ~$9.6k USD.
For a used bonnet say std GTA I would say upwards the number ~$12k USD
For a used bonnet say raced GTA I would say starting from ~$16k USD
Just follow a ratio
valuation of a racing GTA $600k USD
valuation of racing GTA bonnet $16k USD
16/600 = 2.6%
valuation of a std GTA $450k USD
valuation of std GTA bonnet $12k USD
12/450 = 2.6%
I think your assumptions are unrealistic in today's market. And the collector car market is right now soft in many areas. Who is buying a 12 or 16 k$ GTA hood. A collector, are they so discerning. Some maybe. I have not noticed GTA sales at the heights you you use, asking maybe.
Just recently some GTA parts were offered for little more than the repro value but used versus new.
But you are right in that as the value of the car goes up so should the value of original parts.