"I ain't no accountant, my arithmetic is sub-standard, but you have invited comment about your goat milk analysis, and I have issues. Yes you are retired and have oodles of time to play crofter, but there are other unacknowledged opportunity costs.
Dormouse goes on to list the other people involved in this project; in particular Dale who does half of the milking. Her input makes up the bulk of the unacknowledged opportunity costs. However, everyone needs a hobby, even when she's a management consultant.
Dale's input is not a cost to her at all, it is an enormous benefit in terms of : ....fresh milk, early rising, fulfilling vigorous activity, communing with the goats, cheese, yoghurt and kefir from time to time. That's enough I'll stop.
Of course my analysis of our costs only applies to backyard dairying. If the enterprise was expanded to say, 100 goats, assuming a market for the milk, it would not be possible to use a price of £1.55p a litre (Mr. Morrison's supermarket price), we would have to take a wholesale price and of course there would be real labour and capital costs involved.
I don't wish to appear paranoid but there's also the argument for self-reliance in food production. Because the supermarkets have most of their goods in the store or on the road in trucks , "just in time management" , we are only three days away from food riots if the system broke down for what ever reason. Goats, hens, potatoes lots of teabags and a big bag of flour might be quite useful.
Further reading: If you want to learn more about backyard cows and goats find a copy of; The Backyard Dairy Book. Prism Press, Dorchester 1972 its out of print but Amazon and ABE Books will have it.