We add up how much it's cost us to feed and bed them, add the slaughter and butchery costs (remember to count fuel costs) and devide this to give a cost per quarter/half/full pig then add whatever amount onto this you feel comfortable with for your time and work over the time it's taken to raise them.
Don't be tempted to sell at cost or cheaper than cost because you're effectively then paying for the meat that others are eating and it can be damaging to those around you who are also selling their produce (and might not win you any friends
).
Check out prices local to you, see how your intended prices compare and go from there
The thing you have to remember is, if you've got pedigree/outdoor raised pigs, this is a 'niche market' and different market completely to what folk buy in a supermarket - look in local farm shops/farmers markets to get a more realistic guide
HTH
Karen