A good 68-72 351W standard bore block goes for around $300-$400 around here.
I tried to sell a running 351C 4V cleveland with a standard bore a few years ago. I started out asking for $1200. I eventually got down to $800 and still couldn't sell it. I ended up parting it out and got about $800 out of it when all was said and done.
1973 Mustang Convertible
460 D0VE-C Heads w/CJ size valves, Lunati voodoo 227/233 cam, Edelbrock performer rpm manifold, FPA headers, Holley 950 HP carb, C6 w/2500 stall converter, 3.25 trac lock