I kept piglets for 4 years before I embarked on this. I did a huge spreadsheet did a lot of calculations. I breed my own pigs. I know to the last euro the cost of each pig at slaughter and the profit/loss of each pig at point of sale (butchered).
My costs for the rear your owns are kept separate from the rest of the herd. I "charge" 80-120 euros for an 8 week weaner under this scheme. I charge a given amount per month, into which I have factored the forecast price rises on feed. I allow a very small amount for vet bills, hoping that the total from that batch would cover a vet visit to the one. I always have weaners and growers available in the awful event of a loss I would replace the pig. I would offer the "owner" the choice of another pig, or a refund. I guarantee quality pork at the end of the period. My charges reflect the quality of the meat at the end and the guaranteed free range quality of life for the piglets from birth till dinner plate.
So far, and this is in its infancy, have a number of very satisfied customers, and a new batch due to go off next month.
You do have to be prepared for lots of interrupted meals when "owners" turn up on spec. and buy shares in the Kleenex tissue co. around slaughter time.