I would probably replace it, because as lizzzi said I too have a historical tendency to just buy the really cheap microwaves.
that being said, I guess it depends on what the issue is and how quickly you can figure that out, because my boyfriend and I were just talking the other day about how pleased we were that last year we "repaired" our microwave instead of replaced it. our issue was that the little sheet of mica (I think that's what it is) on the inside had gotten a hole burned through it and that somehow caused occasional sparking when we ran the microwave (I'm not a microwave/appliance professional, if you can't tell). I was all "let's just replace it" but thankfully the bf put in more effort. we were able to buy replacement mica super cheap off Amazon, and it was really easy to replace (I assume... he did it). plus now we have extra sheets of mica if we have to do it again!
sorry if that was kind of irrelevant, just sharing a microwave-repair success story that makes me happy :)