Here's the story from 2011:
http://www.dvice.com/archives/2011/0...-takes-ano.php
It says that a 12' x 12' panel costs $6,900. A single lane of highway is 12 feet wide, and a mile is 5,280 feet long. That means it would take 440 panels for just one mile of one lane of highway, which would cost a little over $3 million.
A four lane highway would be more than $12 million per mile just for the road panels. Presumably, installation would be on top of that.