Just off the top of my head I would probably charge somewhere around 20-25 hours labor plus the price of a used engine. Labor typically runs somewhere between $60.00-$100.00+ per hour, I am currently at $70.00. So you are looking at $1700.00 plus at least $200.00 in miscellaneous fluids and parts and pieces plus the used engine. I do not know off the top of my head what they get for used 850 motors these days but I would not be suprised when it was $1000.00-$1500.00 or more. Klaus could be right on the money about the valve and you should spend a little more money and have it narrowed down as to why you have little to no squeeze in that hole and then make an informed decision as a valve job won't be cheap by any stretch of the imagination but it will be a bit cheaper than stuffing in an engine used or otherwise.

Mark