What is the lowest 2006 metropolitan population for a Mexico City, Mexico with a GDP per capita of less than 20,300?
CREATE TABLE "population_and_economy_size_for_latin_am" (
  "rank" real,
  "metropolitan_area" text,
  "country" text,
  "gdp_ppp_2008_billions_of_us" real,
  "metropolitan_population_2006_millions" real,
  "gdp_ppp_us_per_capita" real
);
SELECT MIN("metropolitan_population_2006_millions") FROM "population_and_economy_size_for_latin_am" WHERE "country"='mexico' AND "metropolitan_area"='mexico city' AND "gdp_ppp_us_per_capita"<'20,300';