Figure out what the ads rent for in that area. Discount that amount by 30%, and multiply the result by 20%, and that's what you might offer the landowner as ground rent. Make sure that your groundlease does not take effect until you buy the sign, and give yourself plenty of outs in due diligence. Then get a 60 day due diligence period to see if it's legal, and to see if you can rent the ad space. If everything comes together, then buy it.