41 C.F.R. § 101-28.305
The selling price of a CSC item is an average price which is calculated automatically by the CSC computer at the time the item is ordered. Items stocked in CSCs that are obtained from GSA wholesale supply distribution facilities are input into the computer at the price in effect at the time of shipment from the facilities (this price is normally the price shown in the GSA Supply Catalog). Items stocked in CSCs that are not available from GSA wholesale supply distribution facilities but which are obtained from other Government supply sources or commercial sources are input into the computer at the invoice cost. Due to cost averaging, item prices listed in the CSC catalog may differ somewhat from the sale price for a particular transaction.
41 C.F.R. § 101-28.305