Well, its hard to say? what condition is it in? how many miles since rebuild, what is the compression, etc.
I payed 500 last year for a running flathead 8Rt from a 53 truck. I didnt check compression first and two cylinders had nice cracks in them and one valve seat was rusted up and didnt have any compression, so I learned the hard way on that one.
I would think if you can check compression, etc and its good at least 500 and maybe up from there??? you can always start the price a little hi and go from there?
I'm no expert but have spent my share on flatheads. My personal policy is $100 for a non-running flathead and maybe $200 for a running engine that hasn't been rebuilt.
Remember once a buyer gets it home he has to:
1. tear it down (1st opportunity to find out it is junk) - cheap but dirty and time consuming
2. get it "magged"(sp?) (2nd opportunity to find out it is junk) - $200
3. get it decked, bored, polished, ported, relieved (3rd opportunity to find out it is junk) - $500
4. rebuild it and then start it up (4th opportunity to find out it is junk) - $1000+
It is easy to dump $1000 into a block just to figure out it isn't useable so I wouldn't expect alot of money unless you do step 1 and 2 for me, then I would give you $500 to $800. (not me personally, I have spent too much already.)
No one can tell you what it's worth, until you go thru the steps given above. Most people in your situation sell them for $100 "as-is, where-is" with no guarantee because they don't want to go thru the trouble to figure out if they are really any good. There is no substitute for getting your hands dirty, if you want top dollar.
The values given above, based on the amount of work you do, are dead right in my experience. I recently paid $500 for a bare block that had been disassembled, vatted, scrubbed, and completely mag'd. That included delivery.