Let's approach the issue from a different angle to get a better understanding.
If the 1M is
not put up for sale:
- determining the 1M value is only relevant for the insurer;
- the 1M is apparently a keeper, and, hence, precious for the owner.
Prices quoted for 1M cars put up for sale, should actually not be the
only reference point to determine the 1M resale value. The fact that relatively few 1M cars are put up for sale should, as some tell-tale sign, also be part of the equation (and no, I do not ignore 'supply and demand' principles for determining market prices).
Give this out of the box thought a chance too.