GK - Singapore is reportedly about to sign up..
If the B gets less costly than the C, it will be because the C gets more expensive in even smaller numbers.
Meanwhile, the B survives only through the lobbying power of the Marines. The AF regards Marine Air as a distraction, and big Navy tolerated it as long as it was relatively cheap, which it no longer is. (Quick, can you mention any US non-Marine uniformed military leader who has come out in public and said how vital the B is to a joint campaign?) Financially it is a very tempting quick kill.