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: Jan, Feb, March, April, May, June, July: 250, 160, 200, 150, 285, 170. Cleaning cost $6,450, $4,060, 5,100, $4,100, $6,760, $4,200. How much are estimated monthly variable costs using the high-low method?