In the US, it costs $1 billion for a 17 mile road project where they are completely redoing the highway and adding a lane each way. (http://www.freep.com/story/news/local/michigan/oakland/2016/08/15/i-75-rebuild-starts-monday/88638126/). With the added lane, that makes for 4 lanes each way for 17 miles. This makes 138 miles of one lane. 1 billion / 138 = $7.25 million per mile of regular road per lane.
For this solar project, it is 5 million Euro for 1 km. The picture only showed this as one lane on one side of the road. 5 million Euro equals $5.22 million USD per mile. To go from km to miles, we get $8.4 million USD per mile of solar road per lane.
Further breaking this down, it is about 16% more expensive to construct a mile of solar road than it is for regular road. Not bad.