If it's original my Blue book says anywhere from $500.00 to $225,000.00 depending upon condition and features. That's gonna be one heck of a hard value to guess at without a lot more details including some pics.
As an afterthought I watched an 1892 rifle, octagon barrel, full length magazine that had been reblued and refinished, with a mag tube that had some serious big dents in it (looked like they happened after it was refinished) sell for $1550 this last weekend on an auction. I doubt a complete 1873 no matter what condition would sell as cheap as $500.
TDF