The materials cost on the box repair aren't that expensive, it's the labor. I am in the midst of rebuilding the frame that holds up my waste tanks, as well as the galvanized boxes, along with all the other associated rear end rotted out repairs. Right now my total material costs, not including a new rear aluminum channel are still under $200. By the time I get done I will probably have at least 20-25 hours of labor in it, including the disassembly, welding and reassembly time. I am not a professional Airstream shop, but I have done plenty of restoration and repair on a variety of things and I suspect my time isn't going to be too far off from what a pro would put into it.
I dearly love the older Airstreams, but the only way I can afford one is by doing my own labor. Being highly mechanically inclined and owning most of the necessary tools is a plus.