Well, Foxtrot was a bit on the high side. I believe the expected cost is actually $80k, which is roughly middle-of-the-road for your high-end sports cars.
I would wager that they are insisting on top quality ones to curtail the potential of overheating batteries (or explosions, fires. etc.). Also I recall that they plan of having each one individually encased for saftey reasons.
Well, besides all the safety measures, that's an awful lot of batteries, and I just think $20k seems a bit low unless you mean the cost to produce them. I'd imagine that replacement batteries will be sold at a significant mark up, so even if it really is only a quarter of the cost of the car, I doubt replacement batteries will be cheap.