Hard to tell.... first, what kind of condition? Mint, used but not abused, or well used?
The big one - .264 mag is pretty rare, since there weren't that many produced (relative to other M77s which were in much more popular calibers). The rarity helps the price, but there aren't that many buyers for a .264, so you need to find that one guy who really wants it, otherwise you would have to price it REALLY low to sell it.
Scope - unless it's top-shelf glass, the scope adds little or nothing to the value. Even if it's a great scope, it may not be what the buyer wants. Take it off and sell it separately.
Not much help in answering your question - just things to consider. If I had to take a wild guess, it could run anywhere from $350 to $800.