That's actually very good value for rib. I think my local farm shop charges about £23/kilo, and that's just for commercial Aberdeen Angus. Rare breed like your Longhorn, and the Hereford that my other butcher sells is usually a lot more.
Last Christmas we were too late to order a rib from either farm shop or butcher - they had pre-sold everything by the second week in December. By some festive miracle, Tesco's had a glut and were selling it off cheap. It was only 21 day aged and of unknown source (other than British), but we bought some and dry aged it at home for another couple of weeks. Wiped off the slimy smelly bits on the surface and it roasted up a dream for New Year