The bike appears to be a Norton ES2. The frame should be from between 1947 and 1952 as it has the rear plunger suspension. The engine and frame numbers would be needed to find the exact year.
I'm not an expert but the gearbox appears to be from a Triumph. And the bike seems to be modified quite a bit from its stock factory appearance.
No idea about pricing, but prices vary from country to country and knowing the location of the bike would help others give an estimate.