user: Question: What is the average winner's share won by Billy Casper?
Columns: ['Year', 'Champion', 'Country', 'Venue', 'Location', 'Score', "Winner's share ( $ )"]
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'real']
Sample row: [2013.0, 'Kenny Perry', 'United States', 'Omaha Country Club', 'Omaha, Nebraska', '267 (−13)', 500000.0]
SQL:

assistant: SELECT AVG("Winner's share ( $ )") FROM "Table" WHERE "Champion" = 'Billy Casper';