I think it's more to do with people gauging the cost of a secondhand sale against the 'current' new price.
When I bought my original MM Jazz a few years ago and if my memory serves me right, I paid approx. £799 for it new. I then sold it for about £450 (50-60% of the cost of new one).
Then I realised the error of my ways and decided it was the only bass that I've ever missed so I bought an Olympic white MM but by then they had gone up to nearly £900.00 new.
Now, I see websites listing them new at over £1K. If you apply the 60% rule of thumb for a s/h one then you can see how they are reaching £6-700 used. (I notice that the ebay one for £900 has been listed a few times and was even cloned at one point so not sure about that one)
As it happens, it's still my main bass ( I changed the pre-amp to a Bartolini TCT) and for me, I haven't found anything else that has come near it.
Cheers,
Jim