The cheapest cost is along the bank which will cost 80 million
How to find the cheapest cost
The cheapest cost is solved considering the described locations
The locations will result to a right triangle
Opposite:100 miles downstream
Adjacent: 60 mile wide river
Hypotenuse: going through water (under water)
Distance along the bank = 100 + 60 = 160 miles
cost = 160 * $0.5 million = 80 million
Distance under water, using Pythagoras
= √(100² + 60²) = 116.62 miles
cost = 116.62 * $1.3 million = 151.6 million