It's not just the HP, increased torque, better suspension set up and the bodykit, which will be wanted by most future 370 owners, especially the wing. Then there'll be the way that this is delivered.
I hear what you're assuming as a premium, but I spent £3k on my 370, that gave me a bit of sound (bearing in mind not many owners have heard the Nismo yet), around the same gains on HP and torque, through an Uprev and "performance" filters, then there was a few quid on suspension, that to be honest was harsh on UK roads and some mudguards, this all added up to be around the same price I paid new that you can get the Nismo version and mine still looked like a stock 370.
Then there are the wheels, well there's already huge interest in them on here, with Octet actually buying a set. The offsets are bigger, may negate the need for spacers that a stock 370 requires to give it a better stance and the rear wheels are also wider.
Gents, all the bodykits and "performance" mods, turbo or supercharger excepted, at HUGE cost, will not give you any more than a stock Nismo.
Then there's the right to correctly display a Nimo badge and not a fleabay purchase.
Also, Will, you naughty boy, the roadster came at a premium also over the coupe, and, I'll say it again, it doesn't even have a roof