Oh yeah, ask any F-35 partner nation how cheap and easy it is to add anything to the aircraft or vary things. He who flinches first gets a massive bill.
Just ask the Canadians how much a probe was going to cost?
Guess why we are where we are with external tanks?
Bottom line, if you want something that the US are prepared to wait for or have another platform to cover the gap then expect to pay. A lot. Of course, even if you want it Uncle Sam can just say no.