I owned Nigel's last 12 for a couple of years and when I sold it via Plans I think it was a sensible and fair price. Since then it has passed through a few dealers hands with the price jumping each time. Of course there is an even higher priced NA2 facelift (LHD) on a well known auction site for 17% more than the last 12. If you want to pay more there is another well known site that has a LBB base NA2 facelift (in Hong Kong) at £140k. They also have 2 NA2 Rs for £260k plus import costs to the UK. I sold a 2003 NA2 NSXT privately for close to £80k earlier in February 2018. I had 2 willing private buyers at this level. For what it's worth my view is the NA2 facelift UK market is £70-90k for reasonable mileage cars, with pre-facelift NA2's very close to these levels.
As for NC1 vs NA1 vs NA2. I do think that the relative rarity of the cars is a big factor, even the most common early NA1s remain rare.
I won't be surprised to see the NA2 and NC1 price close further over the next few years, after all many older Ps and Fs command higher prices than more recent models, and the same market sees new models depreciate whilst old models continue to rise. NA2 NSX are rarer than many so called rare Ps and Fs. Don't forget NA2 in RHD form are also very rare even in Japan, so they will always have the edge on NA1s in the UK and Japan based on rarity.
If sales remain as slow as they appear to be with NC1s eventually they should also rise (due to rarity) but this assumes the technology stands the test of time as well as the NA models. I personally I remain doubtful about the technological progress of batteries despite Tesla's apparent progress - don't forget batteries have been around longer than internal combustion engines, there are both chemical and physical limits that battery technology will never overcome. I think a Hydrogen breakthrough is more likely than the step changes still required in battery technology.
Finally prices in Japan have been rising at least as fast as the UK market particularly for the NA2, S and R types. There are plenty of imported NA1 cars but don't remember seeing any base model NA2 imports in recent years. Low mileage NA1 R prices seem to be over £150k and NA2s are certainly over £200k with some over £250k. These are Japan prices so add 30-35% to those to get on the road in the UK.
The other factor to consider is with less than 500 UK NA series NSX it only ever matters what value 500 people place on the cars. Thankfully there are more than 500 people that can see past the badges of the more obvious and more common alternative cars. I have lost count of how many P and F owners ask me why I have an NSX and not a P or F, when I take them out in the NSX most are surprised how good it is, some say they would have bought one instead had they known.
This is just the unashamed biased view of a fan of all NA series NSX and internal combustion engines in general.
Neil