Ball park no. Leaque size maybe. It also depends on the size of the board. For example one, two or three bulletin boards (the big one) are usualy sufficent. For the smaller boards (used to be called 40 sheets) you buy by the percentage of market you want which translates in to the proper number of billboards per market.
Okay so for a bulletin, if you can buy one on a six month contract(usually the minimum, with 12 months prefered), you'll pay anywhere between $750 to $27,500 per month depending on the market.
For the 40 sheets and for reaching a 40% share (a rule of thumb minimum) and for a six month contract, you'll pay anywhere from $2,000 to $200,000 per month depending on the size of the market.
I would suggest you find out who does boards in your area and call them. Billboard companies are always happy to provide rate sheets and information.
Also, I thought about this, but haven't done it yet myself...
Chalk on the sidewalk. You can go around and draw your webaddress on the ground at crosswalks prior to rush hour. From my understanding, because it's chalk and water soluble, it's not vandalism and therefore... legal.