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 250 160 200 150 300 170 260 Cleaning cost $6,450 $4,060 $5,100 $4,100 $6,880 $4,200 $6,530 How much are estimated monthly variable costs using the high-low method?