It might well be worth $850, but you should remember the cost of feeding it is not low. Short 16 ga rounds (From RST) go for about $11-14 a box, and 9.3x72R seems to retail for several bucks each time it goes bang.
Assuming the bores are good - something you can see yourself - I would say passing a good, thorough inspection by someone who knows drillings would be a prerequisite and should be something you negotiate into the deal with the shop owner.
OTOH, the engraving is substantial and that, in and of itself, is worth something.
