Also, while you are at it, you might try pricing out a 'top spec' Open class car as well... Prodrive sells them as well. Its not cheaper.
(from someone who has driven 5 different Grp N cars in competition in the last 12 months)
Seriously though, we dont have any top spec cars in NA. High spec, not top spec. The main differences are in the subtle areas where big money translates into small gains.
Like, min weight car and shell, blueprinted engine to max tolerances, highest grade fuel, engine mapped for fuel and the event (alt. plus car configuration), dogbox with semi-active differentials, optimized weight distribution. Basically, the 'Super N' specs.
Most of the stuff I've just listed most people will never do to their Grp N cars. Even at the UK events I've seen, there are only 2 cars with the max max going on. In australia, there are about 6 or 7 cars with the max max going on.
Here at home, there was a fairly max grp N car kicking around, that was the one I drove at LSPR. But that was an older spec car. But still 'top spec 1 year old'. I think it listed at asking price of $70,000'. It is widely regarded that a well-driven top spec Group N car should be as fast or faster than 3-4 year old A8 cars.
But like I've said before, 'top spec' Open is still even more $$.
And 'the last 10% costing 75% more' is a 10% that I beleive even if you spent the money on it, as a competitor, few people would be able to take advantage of it. In other words, if you just had a fairly high spec car (i.e. good ECU and a good gearbox) and drove it well, you could probably outdrive the 10% that someone who wasn't driving well but could outspend you gained. Or to put it another way, the only time you would go 'that last mile' was if everything was max max max to begin with. Otherwise, it wouldn't be worth it. But if you were competing against 'top spec' Open or 'top old spec' WRC/A8 cars, it would be worth it, because you would still be spending less but still being competitive.