I'm sure the factory knows what they are talking about on the amount of work to replace a rear spar verses building an entire wing, but maybe they are figuring it if they were doing the work and charging what ever their shop rate is. I don't count my time as $ so if I was doing it, my concern would be how can I do it so it's good enough and at the same time not spend any more then necessary. What I would do is cut the spar off on each side of each rib, then cut lengthwise through the remaining piece of spar material in each rib and peal it out, using some heat if needed to soften the epoxy. Once all the aluminum spar material is removed, I would clean up the epoxy with a Dremel and slide in the new spar. Of course the drag brace bracket rivets would have to be drilled out to separate them from the spar as well. It's been a long day and maybe I'm missing something else as well, but you get the idea at any rate. Just a guess, but a new spar and aluminum stiffener might be about $300 not including shipping.