It seems you're assuming (B) is the cost of an entire battery.
I would expect the consumable portion of the battery other than water would be in the form of replaceable rods or plates much like replacing a spark plug or a Diode.
They may want $15,000 for the entire battery, but the Tesla engineers are pretty bright people, and I am sure will find a solution that's easily maintainable. They must have something in mind so far, or I expect they wouldn't have taken it this far.
But you're right though, all this is at this point is speculation. I for one look forward to discovery and implementation of newer technologies. We won't know what will work and what won't until it's been put it on the market and tested 'in the wild'.
Having been a wrecker driver, I think the idea of a battery that goes completely dead on impact by it draining all it's water is beyond awesome. With electric cars these days just touching one of those things after a major accident could send you to the pearly gates.