This widely praised Nikon 105mm macro lens has a maximum f-stop value of 2.8. I've held it in my hands, it's a big lens. Meanwhile, this 50mm Nikon lens can go up to f/1.2, despite being 25% cheaper and a LOT smaller. So at least with these two lens, there is not a very direct correlation between price and f-stop ability.
What is it that determines a given lens' f-stop range? Why can't that 105mm go sub-2?