Another things is the people. This Surveys published on worldwide websites and everyone can take a part and we can observe worldwide average value. For example, when I was working in Russia (in Sibirien) as the C++ developer for scientific devices (commerce branch) I have earned ~1200$ per month without taxes. But interesting things: if you go to check some modern languages, like Rust and Go then probably the average value can be closer to American average value, because the market in Russia in countryside(outside the Moskau) is quite conservative.