I like how you're so confident that it really is an H code Duster. There's no evidence of any matching numbers, the fender tag is MIA, and the VIN on the dash is hardly proof by itself.
I'm not disputing how much it costs to build one of these cars at all, I have a ton of money into mine and it isn't even painted. I just don't think this particular Duster is worth the $20k they're asking because it's a smoke and mirror show.
This car is just a rusty shell under a cheap coat of shiny green paint. It'll still take another $20k to make it right. The only difference is the person buying this one won't think they're starting with a rusty shell, and they'll already have spent the $20k.