I live in a mostly rural county in north-central Colorado. The county maintains ~250 miles of roads of which only 39 miles are paved (the rest of the roads in the county are state and federal highways).
Outside the few small towns in the county, there's no sewer or municipal water. Everyone has a septic tank and a well on their property that they're responsible for maintaining.
Same with cable/fiber. Unless you live in a town, your options are ~1.5Mbps DSL or ~5Mbps fixed wireless (if you have good line of sight to a base station). Neither the county nor any of the ISPs in the area have any plans to significantly improve services, so I'm really hoping that SpaceX gets Starlink up and running sooner rather than later.
Considering all that, I don't think that your calculated premium of $400,000 per rural home actually applies in most situations.