Each psychiatrist, clinical psychologist, licensed clinical social worker, licensed professional counselor, licensed marriage and family therapist, or other expert appointed by the court to render professional service pursuant to § 16.1-356, shall receive a reasonable fee for such service. With the exception of services provided by state hospitals or training centers, 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. If any such expert is required to appear as a witness in any hearing held pursuant to § 16.1-356, he shall receive mileage and a fee of $100 for each day during which he is required to serve. An itemized account of expenses, 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.