Building a road is indeed costly especially when your trying to build a "new" one. As what i understood they made the estimate base on lump sum method. And if your to break it down that covers:
1. acquiring of private lots that gets affected. (goverment needs to buy these since its impossible to lease them

)
2. Labor (imagine if they have to work 24/7)
3. Materials (prices may vary according to the dollar rate)
4. Renting of bulldozers, backhoe's, et. al.
The bigger the road gets, the bigger the money is needed to construct it.