Since most people looking to buy a tailgate are looking to replace one that already has "rust and dent issues", I wouldn't expect to get too much.
I've seen pretty nice used ones go for $100 to $150 on ebay, with NOS and "new take-offs" going for upwards of $500. I paid $225.00 a couple years ago for a new take-off early Gladiator gate that was perfect, and consider it a smokin' deal.
Depending on the degree of rust and dents, maybe $50.00 and up?
'63 J200 2WD
'63 J200 4WD: J20 Axles, extended wheelbase, factory stakebed
'63 J100 Panel Delivery
'91 Grand Wagoneer Final Edition
"If you're going to laugh about it someday, you might as well start now".