Reg, I agree RUN AWAY. When you factor in cost of new microwave vs. repair, they almost are always better replacing. Plus this indicates why right here in this question on this problem - When one major part goes, it usually means multiple things have fried. I have replaced boards to find that the mag and waveguide are bad, then it turns into a repair that exceeds cost of new microwave. Plus I end up getting stuck with parts or fry new parts and it costs me.
What I do is explain to them that to chase a repair like this might be costly. I usually suggest that if they get a new microwave, then I can install it for them. They get a new warranty, and all new parts. If this is a countertop model, then for sure get a new one (in my opinion). Plus if you start on one of these, by the time you get done it's not worth your time invested. Better to have them get a new microwave, I then charge 125 to install one over the stove. Way less headache.
It was different when microwaves were in the 600 plus price range. But now they are so cheap it's a whole different ballgame. I do occasionally fix real top end microwaves like the convection ones by KA< but for the most part I don't get involved anymore. Too Much Headache. It's also different if you work for big company like sears - you don't buy the parts like I have to so it's less worrysome about the cost of the parts you might ruin. I have to absorb that myself.