The economics really don't work out for the average standard everyday $100 consumer counter top models. The only way it does is DIY. Salvaged magnetrons from scratch and dents or working machines that failed in another manner or units that were removed/replaced during remodel are available from some small appliance dealers for $10-$30.
For commercial units or over the range models that cost a lot more to purchase the economics can work out. New magnetrons can be purchased for most consumer models for $40 - $50. A lot of the postings here are people inquiring about OTR microwaves.
Other situations where people repair is if they have an odd size / built in or top of the line (maybe + convection oven) models.