Each psychiatrist, clinical psychologist, or other expert appointed by the court to render professional service pursuant to § 19.2-168.1, 19.2-169.1, 19.2-169.5, 19.2-182.8, 19.2-182.9, or 19.2-301 who is not regularly employed by the Commonwealth of Virginia, except by the University of Virginia School of Medicine and the Virginia Commonwealth University School of Medicine, shall receive a reasonable fee for such service. For any psychiatrist, clinical psychologist, or other expert appointed by the court to render such professional services who is regularly employed by the Commonwealth of Virginia, except by the University of Virginia School of Medicine or the Virginia Commonwealth University School of Medicine, the fee shall be paid only for professional services provided during nonstate hours that have been approved by his employing agency as being beyond the scope of his state employment duties. The fee shall be determined in each instance by the court that appointed the expert, in accordance with guidelines established by the Supreme Court after consultation with the Department of Behavioral Health and Developmental Services. Except in aggravated murder cases pursuant to § 18.2-31, the fee shall not exceed $1,200, but in addition, if any such expert is required to appear as a witness in any hearing held pursuant to such sections, he shall receive mileage and a fee of $100 for each day during which he is required so to serve. An itemized account of expense, duly sworn to, must be presented to the court and when allowed shall be certified to the Supreme Court for payment out of the state treasury and be charged against the appropriations made to pay criminal charges. Allowance for the fee and for the per diem authorized shall also be made by order of the court, duly certified to the Supreme Court for payment out of the appropriation to pay criminal charges.