NaB - The NAO report is full of references to the operating cost of F-35B. I'm also guessing that the SDSR reviewers were aware of the ructions in Washington over the B, which culminated in the current "probation" phase - and just because the aircraft is STOVLing under test at Pax doesn't mean that the issues which drove that have gone away.
B is at best going to be expensive to operate. There are also numerous fixes in the work to solve technological issues that can be worked around in flight test (such as clutch heating in up-and-away) but are not acceptable operationally. The probation period is to show that those fixes work without adding more cost (that is, mostly in terms of durability and maintenance) or degrading bring-back through added weight or reduced lift.
And how much is it all worth? The Marines are talking about "11 extra carriers" but the fact is that the LHA/LHD can carry six jets (with no EW, AEW or tanker support) before they start pushing helicopters over the side.