Battery pack is supposed to last 5 years or 100k miles. It doesn't just stop working though. The range that you can travel will shorten and that's when the recommend replacement. Kind of like a laptop battery. Not sure on the cost. I know that the technologies keep changing and that over the past 5 years they have come down. I do see that gas keeps getting higher. I'm driving 600 miles a week for work alone. Then there's the pleasure driving and errands and misc stuff.
I am looking at moving or at least staying in a guest bedroom closer to work. It's hard to imagine not having a place in SF. I've been there off and on for 21 years now.