Schriever Corporation is an oil well service company that measures its output by the number of wells serviced. The company has provided the following fixed and variable cost estimates that it uses for budgeting purposes.

	Fixed element per month			Variable Element per Well Serviced
Revenue				$	4,500
Employee salaries and wages	$	57,200		$	1,100
Servicing materials				$	600
Other expenses	$	31,000

The planning budget for May was based on 36 wells serviced, but a total of 31 wells were actually serviced in May.

The activity variance for Servicing materials for May would have been closest to: