For 18k how many haven't been driven for years? Scouts have kind of blown up in recent years so values tend to be high now, but it's all relative. Online auctions have other fees and so the prices aren't perfectly relevant to a private sale.
If you see some rust, there's lots more. Most years for the scouts had no galvanizing (a buddy is super into the). The sheet metal tends to be stupid simple, but that's as much bad as good. Patch panels are easy to insert and hide. Plus, that area you're calling surface rust looks a lot worse to me. It's bubbly and rough. May not be cancerous, but those areas have probably lost a good amount of thickness. Plus the way the joints were done often means the worst areas are invisible until they crumble.
My buddy had a friend who bought a scout ii and he wound up replacing the whole front and rear floors once he dug into it. The visible stuff looked about like your photos from what I recall, and it was a desert car too!
I think with all that in mind, I'd be thinking $12-15. I think $18 would be OK if it hadn't been parked for some years (thinking about tires, belts, hoses, etc) and if there was no real rust except on fastener heads. Just my $.02
