They can be all over the place. I know a guy that just sold two complete 340 long block cores for $1200 a few days ago. Sometimes I see them for $350, sometimes $1500 each. It depends on the year and shape of the engine.
It's a Fair Market Value thing.
See, if I needed them from you, they'd only be worth about $54.92;
however, I had them and you needed them, they'd be priced considerably higher, say around $1750.00
But that's just me...