The Thriving Pharmacist has spent some time in the past discussing MAC prices as they are applied to PBM-pharmacy contracts. The concept of MAC price, however, is also a tool used by the PBMs in their dealings with the payor. Despite being several years old, the article referenced below address how the PBM industry manipulates MAC prices on the payor side.
For an excellent explanation of MAC pricing from a Payor’s perspective, read Don’t Get Caught By PBMs’ MAC Mousetraps by Linda Cahn (ManagedCare September 2008)