I am really confused with explanations given in the RMC and Crowe textbooks regarding FPIF contracts.

From one point of view, these are fixed price contracts that assume additional incentive fee for meeting some objectives. And this implies that buyer should not care what are the actual costs of the seller

But from another point of view, there is a buyer/seller share ratio for cost overruns. So, buyer has to audit those invoices to verify and agree on sharing the costs overruns.