IMK, since this is left out, 'F' actually stands for fraction. The number that follows the 'f' represents the relationship of the effective focal length of the lens to its aperture. For example, F4 really means 1/4 i.e. the diameter of the aperture at f4 will be a quarter of the effective focal length of the lens. In the 35mm format, a standard 50mm lens set to f4 would have an aperture diameter of 12.5mm (50mm X 1/4).

Therfore, the smaller f value will actually means the lens' diameter would have to be bigger and therefore more material and more difficult to produce = higher cost. Using the example above, a 50mm f4 lens will slightly bigger that dia 13mm whereas a 50mm f1.4 would be at least 60mm diameter.

400mm f 5.6 = 72mm dia

400mm f4 = 100mm dia

OEM brand, Canon or Nikon's lens will cost more that 3rd party because they are of "better quality" and 3rd party will need to lower their price to compete with the big brands. If the are the same price, there will not be any 3rd party lenses in the market. everyone will buy original lenses and will save us the headache of choosing which brand to buy.