Just like anything in capitalism, there is a wide range in the quality of the product, hence a wide range in the ways(costs) of growing it..
If you want "Indiana ditch weed", then it is cheap, you want "Panama Red" or "Maui Wowie"(Did I just date myself?) then expect to pay a bit more.
Not that I know anything about growing, selling or smoking pot, mind you...
My friends in California say they can grow it in their kitchen window. ;)