"The cost of billboard advertising ranges from about $700 to $2,500 a month. At that rate, ten billboards could run as much as $25,000 per month. That sounds like a lot of money, until you realize that a full-page ad running for one day in a major newspaper costs about the same."
http://www.gaebler.com/Billboard-Advertising-Costs.htm

see the above link for more info.

Billboard advertising have been around for quiet sometime and even though there is a rush of advertising toward the new media like the world wide web and television, as long as there are busy thoroughfares & highways, billboards will always be there catching the attention of motorists, commuters and the mass market. Billboard is part of the marketing jargon of companies trying to sale their wares to the public and over the years, billboards have evolved over the years with its catchy images and new phrases.

Most billboard advertisers have employed a variety of techniques to improve their craft by employing the latest in printing innovations and unique images. Accordingly, "billboard advertising costs in the range of $1000 to $3000 per month and if you register for putting nearly 10-15 billboards per month, then the cost would be nearly about $30,000. This would appear to be a large sum of money, but an advertisement in the daily newspaper for a day would cost you just about the same."

Billboard advertising ranges from a few hundred dollars a month up to thousands or tens of thousands of dollars a month (and even more than that in certain cases). It all depends on the area that the billboard is placed in and how much your competitors are willing to pay. I've personally seen highway billboard advertised for $400-$500 per month, but some of these weren't in particularly good locations. Getting a large billboard in a big city can be very expensive very fast.

You can also get smaller ads including bus stop ads and other formats that are similar to billboards, but cost less. There are lots of options, so it pays to keep an open mind.

From what I've seen, CBS has a pretty large network of outdoor billboards. If I were you, I'd likely contact them first to inquire about rates.