At my last job of ten years, we didn't have any sort of support agreement, so we called MS Product Support for our servers (Windows, Exchange, etc) on a per incident basis (if I recall correctly, $249 or $259 per incident).
In the ten years I was there, we contacted PSS a total of 13 times and paid for exactly *one* of those calls and they weren't even all because the issue was the result of a bug -- sometimes it was the result of a knowledge base article which was unclear or incorrect. Twice they even made the call complimentary because the program they use to enter credit card information was down.
Generally, in my humble experience, we were never charged if all troubleshooting steps necessary were performed and documented prior to calling. It seemed to me that as long as it wasn't user error or a lack of troubleshooting steps on our part, they generally helped us out for no cost -- and even followed up on fixes when resolution implementation required multiple days.