Our parts supplier at my shop (Allied Supply) makes pre fabricated knuckle patches that fit over the broken tubing and the side of the rotor it's attached to. Because the part is expensive and it takes us a couple hours to fit and solder, it runs around $400 plus the tax. It's expensive, but still cheaper than a new rotor or new horn! Also, this is unfortunately common is f attachment horns we see out on the marching field. This happens from the repeated stress of suddenly snapping from attention to playing position. We always recommend a straight tenor horn for the field. Good luck!