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$20 good or bad on an old gatemarked griddle is dependent upon the buyer. I don't care for them, but there are those who do, and I don't think they'd mind spending $20 on one like this, if in good condition.
Slant logo and a hinge? Not sure I've ever laid eyes on that combination. They're typically small block logo. A picture would be good to confirm. If it is the combination you say, and it's in otherwise good condition, you might do well to pick it up. Of course, you'll probably never find a matching lid, but...