A taxpayer files returns for three years, claiming a total of $18,000 in refunds through usage of the Earned Income and Child Tax Credits. The IRS determines that these amounts are excessive claims to refund and that the taxpayer should have only received $3,000 in total for those three tax years, combined. What is the amount under Section 6676(b) that the IRS can penalize this taxpayer for excessive claims to refund?