On September 10 during Strikeforce: Heavyweight Grand Prix Semifinals, what was the PPV buyrate?
CREATE TABLE "2011_in_sports" (
  "date" text,
  "event" text,
  "location" text,
  "attendance" text,
  "ppv_buyrate" text
);
SELECT "ppv_buyrate" FROM "2011_in_sports" WHERE "date"='september 10' AND "event"='strikeforce: heavyweight grand prix semifinals';