The rule of thumb in Western EU (mind you it is only an order of magnitude and may vary) for a 10 meter wide road (think one way highway two lanes+emergency lan

The rule of thumb in Western EU (mind you it is only an order of magnitude and may vary) for a 10 meter wide road (think one way highway two lanes+emergency lane 2x3.75+2.50=10 or an extraurban road 2x3.50+2x1.50=10) is usually considered: plain road (on terrain) € 10,000,000 x km viaduct/bridges € 20,000,000 x km tunnel € 30,000,000 x km with a variation of up to -15% or -20% in countries where workmanship and materials are cheaper and + 15% or + 20% in the more expensive ones.