Road building

If a domain ruler wants to build a road, not as an abstract improvement to infrastructure within an urban settlement, but from point to point within his domain in order to ease travel -- how would one go about calculating the cost per mile (for different terrain types) and the number of miles per [unit of time] that a work crew could complete?  Or would the latter just be 500gp/day, as with strongholds?

Let's say a 6-mile stretch of road, in Hills.