Sometimes a microwave can be repaired if you know what you're doing and the part is not all that expensive to repair the microwave.
If the microwave is a more expensive microwave then it might be worth repairing but if it's a cheap microwave then repairing it could cost more than just getting a new microwave.
If you know how to fix the microwave and can diagnose what's wrong with it then you can sometimes buy the parts cheap and replace it and may be able to fix the microwave for $10.00 or $20.00 in parts.
However if the microwave has to be repaired by a technician then it would usually not be worth the cost because of the shop repair fees such as $50.00 to $100.00 and then the cost of the parts which would add up to likely more than a new microwave would cost you.
For me personally I have those cheap Walmart Microwaves that I buy for $50.00 and they last me around 3 years and to me they're not worth the cost of repairing.