The amount of electricity that a Microwave will use depends on the wattage of the microwave.
Some Microwaves use 700 to 800 watts while some larger microwaves can use as much as 1,200 watts.
Microwaves can use around 10 cents per every hour of use if your electricity cost is 10 cents per KWH.
But usually you'll be using the microwave for a lot less than 1 hour at a time.
So while microwaves use quite a bit of electricity it's usually not that expensive to use the microwave.
If you use your microwave constantly then you might add around $10.00 to $20.00 per month to your electric bill which isn't all that bad.
But most times you'll use less than that in electricity costs with your microwave.
You could try unplugging your microwave for an entire month and see how much your electric bill drops for the month and that will tell you exactly how much electricity your microwave is using per month of use.