> Generally speaking, what is the cost differential between hanging wires and digging wires?
Orders of magnitude. I actually once worked in construction, specifically in trench digging for German Telekom.
Putting up poles is easy, drill a hole in the ground for the pole, put the pole in, string a fiber on it, done. Even cheaper if you already have poles, all you need there is a truck with a lift. Downside is, unless you own the poles, you have to rent them from the local utility and some of these charge extortion rates for third parties. And there are always drunkards or people with shoddy cars taking down entire series of poles, not to mention adverse weather events and vandals taking pot shots. Phone and electricity lines are easy to fix, you can probably train apes to do that, but splicing fiber still involves specialists.
Trench digging is a whole different beast: you need to pull permits, dig the trench, hope you don't hit something else that's buried, deal with bad soil (sometimes straight out rock) or shit like WW2 bombs, lay down your cable, bury the cable in a first layer of fine sand, place a warning band, bury and fill up that with the coarse soil you removed, restore asphalt/plaster where needed... it takes weeks and multiple dedicated crews - usually you have one crew that deals with cutting up and removing asphalt, one that deals with placing and removing anti-traffic barriers, one with doing the actual dig and backfill, one that runs the cable, and one that restores surface.
There is a third way however, it's virtually "underground poles" - earth displacement hammers. These "rockets" can shoot up to 70m through the soil in one go. Nice shit but needs the right soil to operate and it will cut through anything already buried, including solid water and wastewater mains.