Our butchers charge about 50p/kg too (but sneakily extra for vacuum packing and labelling, so I don't get that any more). They will also make all the odd and scrappy bits into mince, which is ideal for burgers or meatballs - add onions, chilli, tomato puree, herbs and garlic, or try Tunisian spices - shape and freeze. I freeze my whole lambs in joints and 'meal packs' of chops. 4 lambs will not take up much space in your freezer, unless they are a monster breed. From our primitives we get either two gigots, or if they are big two gigots and two small roasting joints ie they cut each leg in half. Plus either two rolled shoulders (or 4 if big and they cut them in half) or boned shoulders unrolled, which you can stuff and roll yourself at home. Quite a lot of chops (we get valentine chops from the smaller carcasses) plus some odd bits of front legs and lots of bits of rib. The ribs I have never done anything with so now we ask for anything which is not front or back legs or chops to be minced and I make burgers, as above. In the larger breeds you might get brisket and scrag end of neck which you can stew, or you could get that minced too.
We shopped around several butchers before we found one who will chop as we want. One particular disaster was a man who sawed up the carcasses when frozen, so the meat had bone fragments in it. Another put all the grotty bits inside the rolled shoulder, so it was horrible when sliced. Still another made big lumpy chops which were not appetising.
For selling, I work out all the slaughter, chopping and transport costs, for each animal then add on enough to cover production costs. I usually sell for just below top-quality butcher prices, but well above supermarket prices. I haven't sold any for a couple of years but I think last time it worked out at about £7.50/kg. Don't sell just the gigot, or you will be left with nothing but the poorer joints for yourself - sell as half or whole lamb.
Don't forget to put the fast freeze switch on the day before you get your meat back, and to turn the meat after 12 hours or so, so it freezes evenly. Also, make sure you have enough large, good quality freezer bags