The total amount required under § 49.153 for a district to purchase attendance credit under this subchapter for any school year is reduced by an amount equal to the product of the district’s total costs under § 6.06, Tax Code, for the appraisal district or districts in which it participates multiplied by a percentage that is computed by dividing the total amount required under § 49.153 by the total amount of taxes imposed in the district for that year less any amounts paid into a tax increment fund under Chapter 311, Tax Code.