Short answer is about 800 lbs commercial feed.
It really depends on what you feed your pigs, the quality of the pigs and even the season of the year and air temperature. Low quality feed means you have to feed more of it. Unbalanced feed, missing key proteins like lysine, mean slower growth and thus more feeding. High calorie feeds gives too much lard. Some feeds make lower quality or off tasting pork and fat. e.g. fish - avoid in the last month.
The cost of feed, and thus how much a pigs eats and what, are important issues. Commercial feed typically runs about 800 lbs to raise a hog to market weight of 250 lbs for commercial breeds. Multiply that times your local feed prices. ex. $0.30/lb x 800 lbs/pig = $240/pig
Costs of keeping a pig break down to:
1) Processing ex. $200 (varies greatly with region and what you have done)
2) Feed ex. $240 (much higher for organic, a bit less with bulk)
3) Piglet ex. $150 (varies greatly with region and quality)
Very distant 4th) infrastructure and other things (e.g., fencing, housing, etc) ex. $25
You can do your own processing and that makes a big savings if it is just for your own consumption. If you're selling then don't skirt the law by selling uninspected. You might not get caught but is it worth the risk? Either take it to an inspected facility or sell live and let the customer deal with it if that is what they want to do.
Always get quality piglets - don't skimp there because runts grow slowly and eat more feed, don't do as well on pasture, have less meat, have more medical problems, etc. You might see truck loads of factory farm culls being sold but do you really want the worst of someone else's piglets? There is a reason they are culls. Buying at auction is similarly fraught. Either way, vaccinate fully and worm if you go with low priced pigs. Pigs that are from pastur Better is to just get quality to start with. You'll save in the long run.
Cutting the feed costs is the big thing to improve. Pasturing makes a huge difference. Depending on the pigs and the pasture you can replace 20% to all of the feed bill. The trade-off tends to be time. Pigs raised without the high calorie grain diets typically grow slower but speed is not the key. The key is to produce quality pork while keeping your costs down. That is how you net a profit, be it for your own family or to sell.
If you are raising on pasture then look at what you're going to do to make sure they get enough minerals, iron, and in particular lysine. We supplement with dairy for the lysine. Our soils are rich in minerals including plenty of iron. Get a soil test.
We have good varied pastures with lots of legumes (e.g., clover, alfalfa) and other forages on them. In the winter we use hay to replace the pasture. (Not all hays are created equal!) We supplement with dairy, primarily whey, which provides lysine as well as added calories. We buy no commercial feed or grains. The pasture/hay makes up about 90% of our feed and the dairy about 7%. This varies with the season and over time but that gives you a general feel for what you can do. We also at times have pumpkins, beets, turnips, sunflowers, sunchokes, boiled barley (free from the local brew pub) and a little expired bread (great for training pigs to load). (Yes, those last two are grain but I don't buy commercial hog feeds and they are part of the last small percentage of diet.) See: http://flashweb.com/animals/pigs
On or off pasture, look around for what is available locally for supplemental feeding. What ever you do, don't feed post consumer wastes. e.g., plate scrapings. There are many ways to find good sources of pre-consumer wastes that make great pig feeds. Pigs were the traditional mortgage lifters on dairy farms, eating the extra milk and whey, turning it into high quality pork and lard. Grain feeding developed as a way of storing and transporting grain. More recently the whole system got flipped around and the mean now drives the system.
I would not suggest you try to do intense pasturing from the start or any other special diet. Start with commercial feeds and then find what you can add to them, beginning with the pasture if you have it. Among other things you will need to learn about reading the condition of your livestock, very important if you're doing anything other than formulated feeds. For pasturing, read up about managed intensive rotational grazing which also helps with leaving parasites behind.
There are people who will say that pigs can't eat grass or pigs can't be raised on pasture without grain but our ~300 pigs laugh at those folk. This is how pigs used to be raised. I agree with the pigs. They convinced me - pork, and money, in the bank for me.
Sugar Mountain Farm
Pastured Pigs, Sheep & Kids
in the mountains of Vermont
Read about our on-farm butcher shop project: