Commissioners appointed from Prince George's County other than those who are full-time members of the governing body of the County, except where compensation for the performance of these duties is prohibited by law, shall receive fifty dollars ($50.00) per diem compensation for each day in which they attend a meeting or business of the Commission for which official minutes are kept or for meetings officially authorized by the Commission; provided that in no event shall a Commissioner receive more than $50 for any day, but every Commissioner shall be reimbursed for his necessary and proper expenses incurred in the performance of his duties as a Commissioner separate from the per diem compensation. Commissioners appointed from Montgomery County shall receive $6,500 per year or, if the Commissioner serves as a member of an alternate of the Washington Metropolitan Area Transit Authority, $10,000 per year. A Commissioner who is an elected official or a full-time employee of Montgomery County may not receive compensation for service on the Commission. The Secretary of Transportation, or the Secretary's designee, may not receive compensation for service as a principal member of the Washington Metropolitan Area Transit Authority Board of Directors.