Breeding & cultivating cannabis is a science and art. (you'll soon realize when you start growing your own plants). The most expensive type of seeds are auto-flowering & feminized. This means they have been bred to be only female seeds & require no special lighting schedule in order to make them flower.
When you buy non-feminized seeds, you have a 50% chance of getting male seeds. Only female seeds produce flower (buds), and male seeds actually ruin the female plants if they are kept together.