Given that it's a VERY basic model, and has only a volume and sensitivity circuitry (analog), I'd say at the most it's worth something similar to the Harbor Freight models. So... about $25-$30 new-ish.
I guess I thought it might be a rare early metal detector. It appears very old and I cannot find any images of it on line. It seems to predate when Radio Shack began to using plastic in its housings and I think that was at least 40 years ago.
You're probably right about it being very old. LOL
Here's one one ebay though, with box and docs, it looks like it's going for $25, buy-it-now. It's not quite the same model, but probably still in the same range of capability/age.
Any working detector, though, will sell for $15 to $20 at a garage sale. They have a novelty value, and they're fun for kids.