What is the average winner's share won by Billy Casper?
CREATE TABLE "winners" (
  "year" real,
  "champion" text,
  "country" text,
  "venue" text,
  "location" text,
  "score" text,
  "winner_s_share" real
);
SELECT AVG("winner_s_share") FROM "winners" WHERE "champion"='billy casper';