Is there a rule that in every country in the world the salary levels should be the same? Also whats the cost of living in SF? London is expensive, but apparentl

Is there a rule that in every country in the world the salary levels should be the same? Also whats the cost of living in SF? London is expensive, but apparently SF is more expensive... http://www.numbeo.com/cost-of-living/compare_cities.jsp?country1=United+Kingdom&city1=London&country2=United+States&city2=San+Francisco%2C+CA http://www.numbeo.com/cost-of-living/compare_cities.jsp?coun...