CREATE TABLE table_39400 ("Year" FLOAT, "Champion" TEXT, "Country" TEXT, "Venue" TEXT, "Location" TEXT, "Score" TEXT, "Winner's share  (  $  ) " FLOAT)

What is the average winner's share won by Billy Casper?
SELECT AVG("Winner's share ( $ )") FROM table_39400 WHERE "Champion" = 'billy casper'
SELECT AVG("Winner's share ( $ )") FROM "table_39400" WHERE "Champion" = 'billy casper'