I'd say its value would depend on what someone would be willing to pay for it.
While I personally think its a very cool old piece, it would need to be a sentimentally motivated buyer to get any money out of it IMO. I've not ever heard of old Sunn consoles being considered "collectable", but there may be somebody out there who wants one desperately and would pay up.
Cool from a nostalgia standpoint, but it's not "magic" like a vintage Midas or Neve or the like...
For example; from a technical functionality standpoint an Allen and Heath Mixwizard3 16:2 can be had used for about $500, has twice the inputs, two built in effects engines, rack mountable at 22 lbs, and is far more efficient and flexible as a "working" desk than any of the old 70's consoles.
If you are trying to sell it, figure out what you would take for it and list it in the classifieds section with a price. No shortage of Sunn nostalgic folks here!
Or put it on eBay with no reserve, you will find out exactly what it's worth that way.