Cost benefit analysis needs to look at all costs for the life of the thing being analyzed. That means not just the benefits to the natives. As far as less costly services the road will allow, there are are even less costly options than a road, such as move everyone to where the services and infrastructure are already available.

"The difference in price of goods delivered year round or easier and less costly services " This is like saying look at the money I saved buying this expensive thing at half price. Just think of the money you would have saved not buying it at all.