CREATE TABLE table_64447 ("Rank" FLOAT, "Metropolitan area" TEXT, "Country" TEXT, "GDP  ( PPP )   ( 2008 )  Billions of US$" FLOAT, "Metropolitan population  ( 2006 )  Millions" FLOAT, "GDP  ( PPP )  US$ per capita" FLOAT)

What is the lowest 2006 metropolitan population for a Mexico City, Mexico with a GDP per capita of less than 20,300?
SELECT MIN("Metropolitan population (2006) Millions") FROM table_64447 WHERE "Country" = 'mexico' AND "Metropolitan area" = 'mexico city' AND "GDP (PPP) US$ per capita" < '20,300'
SELECT MIN("Metropolitan population (2006) Millions") FROM "table_64447" WHERE "Country" = 'mexico' AND "GDP (PPP) US$ per capita" < '20,300' AND "Metropolitan area" = 'mexico city'