You are correct, billboard prices are elastic like anything else, and will probably decline in the current recession.
I have always had good luck lowering the cost to build units through thinking “outside the box” on how to build them cheaper. This includes calling all of the regional fabricators, erectors and sign companies looking for used structures that were torn down at other locations. I also aggressively look at building units with wooden telephone poles, steel I-beams and smaller diameter steel multi-poles – anything to get the cost down substantially.
At the end of the day, a billboard must positively cash flow. You can’t build units that cost more than they earn.