Paul's Cleaning Services provided data concerning the costs incurred to clean hotel rooms for which hotel customers pay $150 per night. Data for the past 7 months are as follows:

January February March April May June July

# of rooms 250 160 200 150 270 170 260

cleaned

cleaning costs $6,450 $4,060 $5,100 $4,100 $7,000 $4,200 $6.530

How much are estimated monthly variable costs using the high-low method?