Rooter'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

Number of rooms cleaned 250 160 200 150 285 170 260

Cleaning cost $6,450 $4,060 $5,100 $4,100 $6,760 $4,200 $6,530

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