Brook's is a specialty retailer of souvenir wear. Currently, the firm offer T-shirts, sweatshirts, and caps. Its most recent annual sales consisted of $18,000 of T-shirts, $9,000 of sweatshirts, and $1,800 of caps. The company is adding polo shirts to the line-up and projects that this addition will result in sales next year of $15,000 of T-shirts, $10,000 of sweatshirts, $9,500 of polo shirts, and $2,000 of caps. What amount of next year's revenue is from side effects of the additional product?