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 Feb March April May June July

Number of rooms cleaned 250 160 200 150 300 170 260

Cleaning cost $6450$4060 $5100 $4100 $6880 $4200 $6530

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