I'm not quite understanding the clevis and chain deal either, the hyd cyl should lift it up and be out of the ground don't know what the chain & comealong does?
But a well used 10 foot chisel plow here will retail for $900 asking, and see them sell at farm sales for $200-500, so your offer would be right on for me. Perhaps with the cylinder might be an extra $50-100 typically that is taken off on the sales around here. But thrn if the shovels are worn down to nothing that would knock the $100 right back off. You'll be surprised how much deeper and harder it pulls when those are new, does a better job.
I was up at the sale near the 'Cities last week, and one brought about $2000, thought of loading mine up and taking it there, but that was a Green sale and it brought out the folks with money I guess. ;)
They had a poor 22 foot field cultivator, well in nice shape but only had 2 wheels total on it, and was older narrower row spacing (no trash clearance) and it brought over $8000, but it was green.
Anyhow, sorry for drifting, I think your price is in the ballpark.