The problem with keeping pigs as you suggest is the traditional ways of topping up their protein intake, and in particular thier intake of leucine, an amino acid they need to survive, are illegal. Wild boar survive by foraging over a wide area, and by eating small animals (including scavenging roadkill). You can feed pigs very cheaply: grass, waste veg and fruit, etc. but they do need some protein to thrive. I've seen one of our sows kill and eat a mixy rabbit! If they can't get enough protein from scavenging, you will have to give them some: even if you are happy with slow-growing, grass-fed piggies they still need the leucine.