What about the cultural differences between London and rural Romania or Hungary? An even larger gap exists there than between SF and rural America, due to the very wide differences in macro national culture, national history and language.

Financially that's also true. The median income in Bulgaria is a few hundred dollars per month. The extreme variance between the countries at the top of the EU and the bottom, is far beyond anything you see in the US between states. There's an eight to ten fold GDP per capita gap between Bulgaria and countries like Denmark, Ireland, Netherlands, Sweden. In the US, there's close to a two fold gap between Massachusetts or New York and West Virginia or Mississippi.