For all voluntary contributions made on individual income tax returns for taxable years beginning on or after January 1, 2003, except those made pursuant to § 58.1-344.4, the Department of Taxation may retain up to five percent of all voluntary contributions made for the taxable year, but not to exceed $50,000 for any taxable year, for its costs in administering voluntary contributions. The amount otherwise payable to each organization for which a voluntary contribution has been designated shall be reduced on a pro rata basis in accordance with the amount of voluntary contributions designated to the specific organization in the previous taxable year as compared to the total of all voluntary contributions designated to organizations in the previous taxable year.