It depends on how much traffic there is on highway where the billboard post is located, the size of it, its visibility (from one or two sides) etc etc. The costs indeed can add up. Make sure you get what you pay for.
It all depends on location...I have never done it, but in my area investors say they go for about $12,000 a month
However, you would be much better off spending $12,000 on highly targeted direct mail...it's better to have the rifle approach than the shotgun approach.