The details that we're looking for are: Is it a tang-safety M77, an M77 Mark II, or a No. 1? 3 different rifles, 3 different values.... all were offered in .375 H&H.
edited to add - I'm not sure which, if any, of the above were offered as "International" rifles (Mannlicher stock), but that would also impact the value.
"A Ruger .375 H&H Magnum" is not enough information.