You can feed your pigs on pasture to get a lower cost diet.
Also, you can get the pig food as cheaply as possible. My best tip for getting cheap pig feed is to buy large quantities of it as soon as you can. It's generally cheaper per ton.
It is important to not sacrifice quality for the sake of cheap feed. You will not help the pig if he isn't getting enough nutrients to grow quickly. You will help your pig grow faster if you provide better nutrition.