I have an Orion Blue Book from 1991, and it lists the retail value of an SX-1980 at $125 or some ridiculously low number. Clearly they were at, or near the bottom of their value curve at that time. Wish I could have found one then!
Of course that was before the internet, ebay, craigslist, etc. By the time I bought mine in 1999 or 2000, I paid about $700 on ebay, thanks in part to a blurry picture and a seller who misidentified it as an "SX-1880". It was not working when it arrived but the seller swore up and down it was working when shipped, so UPS paid for about $250 in repairs.
Even back in 1999 or so, I remember seeing a non-working SX-1980 with no knobs, no cabinet top, that had been stored outside (chassis was rusted!) that went for between $3-400 dollars on ebay.