The $300+ price tags you've found for that thing are typical for surplus resellers who specialize in "rare and exotic" computer parts. Name a given obsolete machine and there's *probably* someone out there who's still using one in some mission-critical application with custom vertical-market software. (Processing data from an obsolete card reader, running an assembly line robot, driving an ancient blueprint plotter, that sort of thing.) Since it does cost money to warehouse electronic junk these sellers depend on panic ensuing when one of those remaining mission-critical machines goes belly-up so they can be there to cater the desperate owners' willingness to spend an arm and half a leg for some otherwise basically worthless hunk of junk in order to get back to work.
(I'm not really criticizing their business model, mind you. The same model applies to auto parts, for instance. If you need a taillight lens for the 1967 Obscuremobile you're restoring you'll pay many times what the item would be fairly "worth" in a purely rational universe to the one guy that has it.)
So really, I wouldn't call the thing a "treasure". If you put it on eBay open for bids you'll probably get practically nothing for it, while if you list it for $300 "buy it now" it's *possible* someone might hit you up for it sometime in the next decade. It's how the market works.
(A DN 5500, which is what it appears the board is for, is a *really* obscure piece of kit. It'd sort of surprise me if there are any still doing productive work, but I've seen stranger I guess. There seems to be *very* little collector's interest in that line of hardware.)
