I'd take an educated guess at $1,000 to $1,200.
Good guess. That's what they sell for around here. Don't know what they sell for on places like gunbroker? Last summer, I saw about 10 of them on the shelves. And many 458 Winchester magnums. But the Whitworth 375's were selling for right around $1,000.00-$1,200.00. I will say that most of them I saw, had much nicer wood on them, than the OP's rifle.