How to Determine a Fair Billboard Lease Rate

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. I have tried contacting the company that advertises on the billboard to find out what they pay, but after many attempts they have not responded. Is there any way to find out the going rates for billboard rental in an area? I know that we should be receiving 15-20% of the rental rate the advertiser pays. But I can’t figure out what the advertiser pays. Any help or advice would be greatly appreciated!