I would imagine that the amount of Sandmans being discovered has probably decreased their value. They clearly made a heck of a lot of Sandmans. It is the supply vs demand that determines their value.
Sandman utes may well end up being worth more than vans as people start to appreciate how much more rare they are to find. Might take 10 years for this to become reality though. Thats about when the ute driving generation will be at the time in life to start restoring cars.