Clickbank wrong initial sale prices?

Sup got a question about clickbank. When you choose to promote a product they show you the average amount of $ you'll make per sale. The product i am promoting is 30 $ per sale. Well, I sold it once today and only got 18 bucks? What am i missing?

Because of the coupons, when a visitor try to leave a website they showed you a coupon. so the price will decrease and your commission will decrease too.