Subject to § 13-215 of this subtitle and subsection (b) of this section, a unit may enter into a procurement contract based on any method of pricing that will promote the best interests of the State. If practicable, a unit shall give preference to a fixed-price form of procurement contract. A unit may not enter into a cost-plus-a-percentage-of-cost procurement contract. A contractor who is subject to a cost-reimbursement contract may not enter into a cost-plus-a-percentage-of-cost subcontract.