Thanks to TS, I knew exactly what needed to be done when my iMac cooked off its power supply last summer. My little local Mac store fixed it for nothing, but said that he did NOT replace the Logic Board, as he thought that it was OK. But a day or so later, zap, out went power supply #2, so when I took it back I said that they had to replace both. At least that is what was advised, so they did. But think of the extra time and one extra power supply that was cooked, that runs into money as you have pointed out. All this at no cost to me, and the store got reimbursed, but if things were done right the first time, it would have been cheaper for Apple.
And the instructions are on their help site.
Jane