Don't even know what the car costs yet.

But, asking such a question is equivalent to asking "what's a new engine cost for this car?" The batteries in this type of car are designed to last 8-10 years or more (often with a warranty to that effect).

If you look at the Prius (and surely the Leaf will me marketed to compete with the Prius), it's hybrid drive system is warranted for 8 years/100,000 miles. A replacement battery pack for the Prius is about $3,000 (retail), but Toyota says they've never had to replace one due to it "wearing out", only from crash damage.

With modern battery technology, I think any argument about cost/maintenance/longevity of batteries is null and void.