Well, the guy my hunting partner got his Blazer from wanted $15K for it. My hunting partner paid a bunch less than that, and we later found cancer on the truck. It had stock suspension and a more or less stock engine rebuild, but it was a 4-speed, and we had both been looking for relatively clean 4-speed Blazers for a while.
So, based on this, I'd say that if this Blazer is absolutely rust-free, I'd be willing to go to maybe $15K on it (if I had the money). It's not worth as much to me being an automatic truck...