Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What is the average winner's share won by Billy Casper?

### Inputs:
table_name: table, columns: ['Year', 'Champion', 'Country', 'Venue', 'Location', 'Score', "Winner's share ( $ )"]

### Response:
SELECT AVG Winner's share ( $ ) FROM table WHERE Champion = billy casper