@Michael Rashfal As someone who has owned billboards I would check to see if the billboard and an easement for access are parceled off with its' own parcel ID, from the property that it is on or next to. If it is not, you are going to have to separate that from the land that it is located on. I didn't check for that and the seller who owned the land went into foreclosure and I lost the billboard lease as it was not a separate parcel for the main parcel.
If the property it is located on has a bank mortgage you will have to get that bank to release that part of the land from the larger part before you purchase the sign.
Also, you are going to have to set a ROI that you can live with on the purchase price. It does seem high, but is it a digital board, how many billboards are in a 2 mile radius, what is the traffic count of the road it is located on -high the better. Does the city have a moratorium on new signs going up. All things you need to find out. Frank used to own his own sign company in TX and knows more than I, check him out.
Good luck and hope this helps.