The ratings are for the pair took me a while to find that information out.
Yes its more expensive than a standard axle but if you do the unibody like you were saying you can go as wide as you want and put the wheels outside the body and not have to do any building around wheel wells inside if you want to go wide.
My camper has a 3500 lb axle on it with a weight of 1440 and buddy it bangs around a lot because it doesn't even get close to loading the springs. Makes for a rather not pleasant towing experience and something long term I need to address and fix and the torsion axles seem to be my answer. Of course I'll just find a trailer they are already on and lessen my pain of then dealing with what to do with the original axles I replace.