A pre-64 built after WW-2 in common calibers isn't worth nearly as much as most people think. But one in 300 H&H is fairly uncommon and should bring a premium. Prices on those are all over the place and the exact configuration and condition mean a lot.
The 300 H&H is a very long cartridge that required either a magnum action, or in the case of the model 70 one that was slightly modified to make it fit. It is a very "classic" round and desirable from a collector or sentimental perspective. It will come close to, but not quite equal 300 WM in performance. Ammo isn't readily available, but not impossible to find either. It'd be best for someone who hand loads.
I'm afraid to guess at a value. But if I wanted one, I'd not hesitate to pay $1,000 if in good condition. And it might be worth a lot more than that in 300 H&H.