The first engine - turns over but unknown - I'd offer 100. I've paid 250 for one like that, but figured I was generous. Really depends on supply and demand in your area.

You mentioned the block/heads come with lots of extras. In my opinion, it's only worth 1500 if it has a ton of aftermarket goodies. (Alum heads, blower, alum intake with a pair of 97's, etc, etc.

I recently gave 350 for a cleaned and magged standard (not overbore) block that the guy will guarantee. That's on the high end of what you could find one for if you shop around. Follow ebay sales for awhile - they tend to be higher than swap meet prices, but will give you an idea. Don't look at beginning prices, but at what things actually go for. There are idiots on there with no grasp of reality.

Flatheads aren't particularly scarce - you just have to know where to look. What you're finding is a revival of old-school hotrodding and folks are digging around in their outbuildings for that old engine that was left out there 'cause they couldn't give it away 30 years ago. AND they think it's their "pot 'o gold at the end of the rainbow" now.