I paid $500 for a bare block that had been tanked, inspected, and the small cracks that were found fixed, essentially a guaranteed good block. I also paid $500 for an engine that turned over easily, was complete, and had the heads off and the deck scrubbed for visual inspection of the deck (which looked good). It turned out to be a POS, had 6 stuck valves, and a crack on the side of the water jacket. You just never know.
Even seeing one run is no guarantee, because many of them with cracks get sealed up with rust. Soon as you hot tank them and start boring, there ya go...