I believe the only people interested in them would be geeks who collect the old DEC hardware they work with.
If I remember right, I donated my keyboard to one such collector for the price of shipping.
(hint: If you are a vintage keyboard-collector, find one in person and ask if they have another keyboard in
trade: you might get something interesting ... or you might get a dud. Either way, it is more exciting
)
In general, you can often find the going price for something by searching on ebay for
sold items went for.
I checked for LK421: there was none right now, but a few full-size boards for DEC terminals went for $20-$50 or so.