If I'm not mistaken, the Nordschleife has 22 or 23 km. I think you should ask Mr. Gustav Eichler, its designer, for a good estimate.
Its construction as a modern road involves important earth movement, I assume. It has a puny 9 m wide for most of its length.
A rough estimate for the road
, without major earth movements (something I doubt) would be around 50 million. Not much, compared with what a car costs.
Throw in another 10-15 million more for widening and I don't know how much for earth walls (the hills are steep), because I don't know what the soil is made of, nor how steep can it be shaped without sliding.
That's without stands, pits, bridges, boxculverts or anything else, of course. A bridge costs between 50 to 100 times as much as a road of the same length, so I don't know if you would contemplate long ones to straighten some curves.
I don't know if drivers would like it. You have to become an expert to learn 22 km of curves
, in hilly terrain, compared with the 12-15 curves of a modern "flat" track where, after a weekend or so, you know most of the braking points by heart.
I think that was the reason why geniuses (or madmen) could take so much advantage in so few laps at that track. Prior to 1971, the Gesamtstrecke had 174 curves! After 1971 some curves were elliminated, I haven't counted them.
Safety run-off areas are impossible to build (or, better said, extremely expensive) for most of the track length, so we probably are talking of improved barriers at many sites, NASCAR style. I wonder what kind of barriers you should build to stop a car in a couple of meters... guaranteeing they won't break, allowing the car to pass through and
also guaranteeing that they won't break your neck if you hit them. We're talking, probably of ARMCO barriers only, modern barriers in that length would be truly expensive.
To get a better idea, try to imagine building an Autobahn in that kind of terrain, and Autobahns certainly don't have 300 kph design speed! Braking lengths, vertical curves, almost everything increases to the square of the speed.
The safety posts for marshalls and the coordination of the track, the extraction of drivers by helicopter, the ambulances, the side road (how would you build it?) for emergency vehicles, well...
Just the location of paramedics or (several, I imagine) safety cars would be complicated to design, to say the least. How many TV cameras would you need?
Imagine trying to design the sites for overtaking... Scot would be furious about it.
Overtaking, I imagine, would be minimal, or we start to talk of serious bridges to straighten the thing.
Everything is three or four times what you need for a current track (maintenance, personnel, resurfacing), so tickets would cost three or four times more. I don't know how many people would pay 2.000 or 3.000 dollars for a ticket... You could construct more stands, true, but you have to pay for its construction (on steep hillsides).
Besides, with such huge laps, spectators could go for a beer and come back before watching a car again.
So the specs I would ask for include, besides the things I've mentioned, mainly naked pit babes (or something! Mosley fighting Montezemolo and Dennis in a cage, chained to the neck...) to entertain you while the cars pass again.