Fair Market Billboard Lease Rates Near Winchester, Virginia

We own a billboard near Winchester, Virginia and need to renegotiate the lease. Can anyone share how to go about finding out the fair market value of a billboard in a certain area/region? I know that our rate is extremely low ($1,000 per year), as my predecessors had let the lease run out and continued on a year to year basis never raising the rate. Thanks for any help you can provide!