Sausage its typically made out of the cheapest meat, so it's always good for home economics.
But I don't understand why prices differ by animal. Where I live, deboned chicken thighs are dirt cheap, and they make the kind of chicken sausage my wife likes best. That's the cheapest meat available here.
Next up is pork, a bit higher in price. Beef, even cheap beef, is more expensive. Finally, lamb, which I love, is almost prohibitive in price. I would make merguez more often if I could get good lamb for less.
Discounting differences in cut, I think we can say, at least in America, that:
Chicken < Pork < Beef < Lamb.
Why is that? Does it cost that much more to raise a cow than a pig?