It depends on standards of living in various parts of the world. Blockchain technology will raise the quality standards over the whole Globe, so the gap between the countries will get closer and we will be living like one happy Human family!
As I am from Bulgaria, living now in Russia, in order to be ok with everyday expenses and having an open option to travel abroad at least once in a three months, I guess I would need about less than 1 BTC per month as per the current BTC price.