Assume $100/kWh, and a 50kWh battery, that's $5000 to replace it, after, let's say 100,000 miles. You've saved, let's say, 10 cents per mile on fuel, so that's $10,000 saved.

Battery should last about a million miles though, with proper temperature and charge management.

Once your battery has degraded to, say, 75%, now you have a free 35kWh battery to store power in your house, that's worth $3500