The largest single expense in raising pigs is the feed bill. It takes about 1,000lbs of good quality feed to go from a 40lb weaner pig to a 300lb market pig, and feed current sells for $300/ton in this area. So if I were to feed the pigs solely a prepared feed, I'd have a cost basis of the piglet cost + $150, or somewhere around $225. Add to that payment for the land, tractor fuel, repairs, fencing and labor, and a small producer can end up with a hog that cost them $350-400 to produce. Since a side of pork in this area sells for $240, for 6 months labor and expense, you only earn $50-80.
That means that you'd have to produce something on the order of 300 pigs to make a meagre living of $24,000 a year. And selling 300 pigs retail is a massive task. If you can reduce your feed costs by half, you reduce the number of pigs you have to sell to make a living -- reducing the feed cost by half means your cost drops by as much as 30%, and that means that instead of $80 a pig, you're making $155 a pig, and the picture looks a little brighter. If you sell 300 pigs at that margin, you're knocking down $46.5k a year -- which is a low, liveable salary.
The pigs are on pasture. They find stuff to eat all over. Plants, animals, mushrooms, they spend quite a bit of their time rooting around looking for things to eat, even when they've got a full feeder. They enjoy foraging.
Spent grain, brewing byproduct
The picture above is spent grain from brewing beer. To make beer they combine the grains they're going to use (wheat, barley, etc) and then boil them in a big kettle. The liquid is then drained out of the kettle and used to make beer, and the solids, basically a calorie-reduced grain porridge, is part of what I feed to the pigs. In this picture there's cans from two different batches of beer. One of the batches they roast the wheat until it's a little burnt, and that's the darker band in the center.
Juice and wine making byproducts
I've talked about apples and wine pomace (leftovers from brewing wine and making apple cider here.
Expired produce and dairy from a variety of sources
and finally, I use formulated hog feed, which I talk about here.