The thing that I question about this is that it means that roads have by
far the greatest value in the mountains, which is also (coincidentally I
guess) where they are the easiest to build. Do we really want all of
the mountain (and adjacent) provinces roaded, but no others?
If roads are to provide a greater benefit in the mountains, then perhaps
they should always take 100 stone to build, but varying effort:
500 effort in plains,
1000 effort in desert,
1500 effort in forest,
2000 effort in mountains.