The two sides will share equally the costs of all known retrofits needed for the aircraft, while any newly discovered changes could result in higher contract costs, the Pentagon said."
Like the saying goes, the devil is in the details.
This could potentially be a problem with so many issues unresolved and so many things left to test.
As for those who wonder what the engines costs.
Pratt, DoD Reach Agreement on F-35 Engine Lot | Defense News | defensenews.com
About 25-30 million a piece on average ,a bit more for the B version, a bit less for the A+C.