I'm sure Frank will have a better response than mine, but when I looked at a property that had a sign on it, I had a friend who owned a business call the billboard owner (Clear Channel) and ask for the going rate.
It was a sign on an interstate highway, two sided, telephone pole construction, vinyl (no digital side) and the kicker on this one is that the land had been annexed into a town about 4 years before and they no longer allowed new billboard construction in city limits. The rates were $2,400 per side for every 4 weeks. They let my friend know if the contract was for 6 months or longer, they could offer a discount. They then sent him a list of other sign locations in the area with the rates.
When in doubt, ask the local sign owners what the current rates are for their signs in the area.