Unless the funder gives different instructions, it is always best to use the guidelines provided by the U.S. Office of Management and Budget in Circular A-110. This requires that you value volunteer time at the rate your organization would have to pay someone to do the specific work performed by the volunteer.
Here's the excerpt from that guidance. "Volunteer services furnished by professional and technical personnel, consultants, and other skilled and unskilled labor may be counted as cost sharing or matching if the service is an integral and necessary part of an approved project or program. Rates for volunteer services shall be consistent with those paid for similar work in the recipient's organization. In those instances in which the required skills are not found in the recipient organization, rates shall be consistent with those paid for similar work in the labor market in which the recipient competes for the kind of services involved. In either case, paid fringe benefits that are reasonable, allowable, and allocable may be included in the valuation."
But always read a funder's instructions carefully. Even some federal agencies will provide a figure that must be used for valuing volunteer time.