It depends on the shape.
How long has it been sitting?
Does it leak oil or burn it?
Has it been repainted?
Does it run? Is it 100% complete?
Is the clutch good? Clutch release bearing?
Is the tranny still good, does it leak?
Is the carb there/been rebuilt?
Is the block cracked anywhere or leaking coolant?

There is a lot of stuff that can hurt or help the price. I've seen seized/not running flattys sell for $20. I've also seen rebuilt ones, painted and restored, go for just under $800.