We eat a fair amount of mutton.
With regard to pricing there are two ways to look at it -
As LouiseP says, it's taken a while to rear to that stage, and other costs have accumulated etc, so should maybe be more expensive then lamb.
However, the vast majority of mutton is a by product, rather than specifically reared. I.E It's a cull anyway, so you are making use of a useless beast, which quite possibly has had some prior out put (i.e lambs) for you.
As for 'no proper market'. . . . . . try telling that to the 1000's of cull ewes that go through markets, all of their buyers and the entire islamic community!