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	$6,640	$4,200	$6,530

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