Nearly all aspects of infrastructure build out are heavily weighted towards labor costs. A 2009 rule in the US required large projects use union labor, and just that marginal increase in labor costs increased infrastructure costs by around 15%. Anecdotally, I had a home addition some years back. Materials costs about 20%, labor about 80%.
Also once built, the capital expenditures for something like broadband drop to negligible rates while the labor cost of maintenance & administration just keeps on going, maybe not as high as the initial build but you need new & different workers too-- sales reps, account managers, etc. Given that prices tend towards the marginal cost of production over time, in this case being mostly labor, it makes perfect sense that a country with 1/8 the labor costs could offer a service at 1/5 the cost in the US, the difference in those proportions accounting for the marginal capital costs of maintenance.
[0] https://www.telegeography.com/products/commsupdate/articles/...
No, this is incorrect. GDP is a proxy for production in nominal terms. If it were just a proxy for local price level, then every place would have the same real income and would differ solely by relative prices. That the U.S. has a a GDP/capita that is 4x times some other nation does not mean that prices are 4x lower in the other nation.
When you are asking "how much would it cost to do something in B than in A", workers in B might be paid 4x more than in A, but you may also need 1/4 of the workers to do the same job. So the issue isn't wages, it's output per unit work, primarily because of accumulated human and physical capital, as well as things like infrastructure, legal system, needing to pay bribes or efficiency wages, etc.
I kinda' doubt that $50/mo is really unsustainable, especially once the infrastructure is already built.