5 C.F.R. § 304.104 – Determining rate of pay.

(a) The rate of basic pay for experts and consultants is set by administrative action. The head of an agency, or his or her designee, must determine the appropriate rate of basic pay on an hourly or daily basis, subject to the limitations described in section 304.105.
(b) The head of an agency, or his or her designee, shall consider the following factors in setting the initial rate of basic pay for an expert or consultant:
1. (1) The level and difficulty of the work to be performed;
2. (2) The qualifications of the expert or consultant;
3. (3) The pay rates of comparable individuals performing similar work in Federal or non-Federal sectors; and
4. (4) The availability of qualified candidates.
(c) An expert or consultant appointed under 5 U.S.C. 3109 may be employed without pay, provided the individual agrees in advance in writing to waive any claim for compensation for those services.