In the car business, after a customer asks the salesperson "What's this ONE go fer?". And no matter what the price is, the customer always whistles! Hence a whistlin' gopher!
"So Mr. Salesman whats this one here go fer?" "Well, Mr. Customer, we are asking $9900 for this particular vehicle." (whistlin sound) followed by "thats too much" hence the whistlin' gopher.