You are thinking of advertising Microsoft products on a sports telecast. As you buy more ads, the price of each ad decreases as shown in the following table:

Number of ads	Price per ad
1-5	$ 12,000
6-10	$11,000
11-20	$10,000
21 and higher	$ 9000

For example, if you buy 8 ads, you pay $11,000 per ad, but if you buy 14 ads, you pay $10,000 per ad. Write a formula that yields the total cost of purchasing any number of ads.