I use my microwave only for reheating food,at 75-80% power (450w-600w),and only for 10-12 minutes everyday. Is it going to case high electricity bill? Any idea what will be the extra unit?

NO.

I’ll go through the numbers. 600 watts x 0.8 = 480 watts
12 minutes is 1/5 hour

480 watts x 1/5 hour = 96 watt-hours
96 watt-hours per day x 30 days/month = 2880 watt-hours/month or 2.9 kW-h per month

Typical price is 10 cents per kW-h
2.9 kW-h per month x 10 cents per kW-h = 29 cents per month

pretty cheap, 29 cents per month