Pontius - you overlook the details that
1. The MoD owns the design (because it funded it) and so gets a license fee on every foreign sale.
2. The RN Type 26s have yet to be built, so the unit price when they do will receive the volume cost benefits of the RAN order.
3. UK contracts for large MoD programmes are "open book", so the MoD is able to see and verify that these cost reductions are applied - it's not just blind faith.
PDR