[Dreadfulnaught]

I hate broken and (more common) stripped truss rods. There is no real reason for them. It’s idjit damage. They don’t damage themselves, it’s usually done by a kid who does not know what truss rod adjustment is for and overcranks it. I have a friend like that. They generally break where the adjustment nut is, not in the middle of the neck or something.

Stew-Mac makes a tool to repair a broken/stripped rod, though I don’t know how easy it would be on a Martin with the adjustable end inside the guitar. You first chase the threads to get them good and clean, then if necessary there is a bladed tool that pockets the wood around the rod so you can put a few threads on it and then install a new nut (or reinstall the old one).

Did you verify that the rod actually is broken? If the nut goes round and round with little resistance, it may be stripped. Did the tech hand you a broken end with a nut on it? If you do this repair yourself it may yet be a bargain.