Road costs are per hex. Factors that influence the cost are listed on the work site's info window.
Generally, roads through worse terrain are more expensive; your road is close to double the material cost of an asphalt road through a flat, clear hex, for instance.

In the mod settings, you can turn road costs down as low as 10% of their standard cost, though adjusting that doesn't change the cost of any work sites in progress.

Also, costs are discounted if you build a road on top of an existing road, so building a dirt road first and upgrading it later would save on material costs. Asking nearby friendly settlements for construction assistance can really speed things along as well, and they bring their own materials.

Do factions provide help for the entire run, or do you need to spend the Goodwill cost for each tile? Also, what affects how much help they provide?

@Revenant, factions provide a set amount of work, based on your negotiator's skill and a little randomness. Goodwill is spent once the help has been provided. If a tile is finished thanks to a faction's help but some work is left, it will go to the next tile and so on until either the work they provide is exhausted or the road is finished.

I can't leave comments on main mode page due to lack of privileges, so I leave it here.
I start use this mode and meet a lot of difficulties at the beginning.
1. First of all you can't see the recipe of different kind of roads until you start construction. So I had to write it in the notepad (funny).
2. Then I try to make stone road I see the iron ore icon, but I remember you need steel alloy (I check it on the main page and also saw people talk about steel). It's difficult to check the list of ingredients until you memorize, especially for people with poor English skill (as myself) and different modes with a lot of materials.

3. Mode compatibility: I use HSK main modes assembly and almost none of my stone types I bring to the construction yield doesn't count at all: Slate stone, sandstone and so on. Those types used to be in the main game, so I think this mode doesn't work with all type of roads except dirty roads.

Update. Mode working fine with HSK (at least with dirt - stone roads).
You need ore + stone together (at least slate stone + iron ore) to start working on stone road.
It would be great if this material would be signed and you can see the list before you start working.

Anyway thank you for the useful mode.