Magnetrons are quite expensive new.
My old microwave self-destructed itself. I took it apart and kept the high voltage transformer for experiments. I had no use for magnetron, but before I chucked it, I put it on eBay just to see if anyone buys. Someone bid it up to $15 + shipping. I'm guessing there are some expensive over-the-range of built-in models that use the same type.
From the buyer end, there's a certain satisfaction in needing a part, then typing in the part or typing in the equivalent part # and have it come up right in front of you at fractions of new price.
"just chuck it"...is the reason we're losing skills to troubleshoot.
Look at the schematic. Usually, microwaves are stupidly simple. It's simply a vacuum tube fed through a single stage voltage doubler. The capacitor size limits the power. Most microwaves have a pair of switch. NO and NC and the latter being the suicide switch. When the door is closed, NC opens first, then NO is closed. If NO is closed first and microwave is activated, it will short across the power line.
http://031d26d.namesecurehost.com/mwd/mv1526_sm.pdf
Mine had a door timing mishap. It blew the fuse, but the short circuit also vaporized the traces on the main board that connects terminals to relay and welded the relay contacts.
On some microwave, you can bypass the suicide switch and its perfectly safe since you have NO still in place, but some newer models requires proof. It must see the NC is closed, then opened, or computer won't allow power up.
Microwave 'engine' still worked fine when directly wired, but it blew the most expensive component (the main board), so I sold the magnetron and glass plate, then scrapped it.
If you're getting voltage to PRIMARY side of high voltage transformer, there's something wrong with the microwave generation system. if you're not, its the controls.