It would be nice to think that $50k+ would be achieveable, as it would cost more than this to do one right. If it was restored properly, and as original as possible it may well see that. However it would have to be a HQ to get that sort of price, and probably be an exception due to what it is. Totally original untouched HZ's struggle to go over $25k, but that is HZ, HQ's are far more desirable, rarer and collectable than HZ's (not just in Sandmans). Just think what a yellow/black L31 M21 HQ van or ute like Greg's would be worth in original Survivor condition like the HZ in Opaline blue at Echuca and that is your top dollar example.
The above reason is why i'd always encourage any of you to not be too anal in restoring them, as this costs too much and takes too long, and you never recoup on your investment when you have to sell it (never say never, it happens to all of us). Use as much good secondhand stuff as possible, and use quality repro bits where need be like Jason's stripes or Rare Spares consoles or Greg's HQ fascias. A half-hearted attempt at being anal will get you nowhere, just do your best with what you have. If you want to do a concours resto go for it but it will cost a fortune! Talking to Ben at Seven82 motors, they have customers spending $50k on chequebook Sandman restos and they are not doing them close to original, and will never hope to recoup what they spend - good for us I say, we can all buy them when they decide to sell and use them as everyday drivers!