It varies widely. I once got in a bidding war for a 70's Chevy panel and jacked that price way way up. I was bidding out of spite, there wasn't any way someone would be willing to pay that much, or so I thought. I gave up at $150 or so. I later bought one with some minor dings for $50.
A while after that, I sold my really nice 70's GMC panel for $75 I think
The 80's panel seems easier to find, but I'd guess anywhere from $75 to $150.