I'm cooking a leg of lamb now from my local Safeway. It costs $7/lb, which is the same as chuck roast. But it's maybe 1/4 more expensive due to that big leg bone (which my dog values highly). Still that feels like the price is comparable to beef, but a nice alternative flavor.
In my location, lamb is closer to $10/lb (at grocery stores - higher at butcher), while I can get (grass fed, super high quality) beef straight from the rancher at less than $4/lb.
I haven't seen mutton for sale at any grocery.