If it's original condition than it's worth way more than $850. I'm guessing it's been refinished, as others have mentioned, and new grips can be had for around $20. $850 might be a reasonable or even cheap value for one in decent refinished condition.
No one will be able to give an accurate value without much better pictures and a description of how it functions. Are you planning on shooting it? If so you might as well pass. There are plenlty of guns from the 1800's you can buy and still shoot, however this is not one of them.
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