user: Question: What is the total number of 1st prize ($) that has a united states country with a score lower than 271 and in a year after 1964 with a winner of bobby mitchell?
Columns: ['Year', 'Winner', 'Country', 'Score', 'To par', '1st prize ( $ )']
Types: ['real', 'text', 'text', 'real', 'text', 'real']
Sample row: [1972.0, 'David Graham', 'Australia', 278.0, '−6', 30000.0]
SQL:

assistant: SELECT COUNT("1st prize ( $ )") FROM "Table" WHERE "Country" = 'united states' AND "Score" < 271.0 AND "Year" > 1964.0 AND "Winner" = 'bobby mitchell';