Only my opinion but....I reckon that prior rego history on a Eureka that enables that car to be driven like any other car has to be worth about $10k. Getting a car on the road these days via the Engineering Certificate route can add up to heaps of time and I've spoken with people who've spent $20k restoring a car without proof of prior rego, and still find they'll have to spend another $5 or $10k to get it engineered.
The engineering option is like everything else in that it's only going to cost more in the future. The value of existing registered cars should therefore only go up if I'm right.
The biggest trouble at present is that cars are too often sold in 'desperate' circumstances, never a good way to sell anything, and as a result, for a hell of a lot less than these cars are worth.
Like anything, no matter what condition it's in, it's only worth what someone is prepared to pay for it on the day. It's just that on most days the value of existing rego and need to sell are not being reflected in our prices. Slug.