I am not sure if today's values are a good yardstick. If you make your money selling eggs you don't want to sell your chicken so cheap that people will be better off raising their own chickens than buying the eggs your chicken produce. With farm animals in medieval times you are not just selling animals - you are selling someone the means to produce their own eggs, milk, cheese, wool... Make that too cheap and you are selling other people the means to compete with you for small money.<br/><br/>My first impulse would be to value goat, sheep, and pigs at about the same as a donkey or mule, i.e. about 8gp. A cow might go for the same price as a pony (30gp), but lets make that 24gp, to have a nice, easy comparison between the smaller animals and a cow (i.e. 3 small animals = 1 large animal). Still means an average person might only be able to buy one or two goats a year, or maybe a cow, but I think that's kinda close to how I want the economy to work.