Start with bag seed if you want to dick around. But sooner or later, your going to want to buy some good seeds. If you just shop around a bit, you can get a some good genetics for $25-$30 (10-pack of seeds). If you want the shit that wins awards, you'll have to spend $120-$180.
Auto-flowers are pretty good for the money and go for around $30. You just can't clone them. You can still let a male pollinate a female and get a bunch of seeds.
Since we're already in winter, if you're goal was to get growing right now you'll have to spend some money on lights, a timer, and all the misc shit you need for a proper indoor setup. But if it's still warm where your at, you can probably put an auto flower outside and still get a decent harvest.