On any car of great worth, I believe that it should be insured comprehensively, rather than at market value. A GT-R will be such a car.
On a car worth less than $10,000 but above $4,000, I might be tempted to insure it at market value - but maybe not.
If the car is worth less than $4,000 I might only pay for third party property insurance.
Sum Insured/Premium ratio depends on too many variables.