> how much would you have to spend to keep a dirt road serviceable?
You're kind of underthinking it. Keeping a road serviceable for regular traffic, especially when taking into accounts vehicles like wagons, is a lot of work. Some decent rain will turn that road into mud when there's heavy traffic, and suddenly you barely have a road anymore.
If you have nothing better to do with your time, I could actually recommend The Pursuit of Glory, which points out proper road maintenance as one of the core pillars of Europe entering the modern age.