We've had several threads on this recently if you want to try the search feature. Basically it varies with each species and depends on what region you are in, what housing and equipment (fences, barns, etc.) you already have, etc. Cows are the most expensive- biggest, eat more, poop more, make more milk. Sheep and goats are smaller, eat less and different things, harder to fence.
For cows a great place is the Family Cow Message Boards
For cows a great place is the Family Cow Message Boards