I've heard the cost is $6500 installed, but that's not much less. If I'm not mistaken, Ford claims to not have had a battery pack fail yet. There are linksin this forum to articles featuring FEHs with 200k or 300k+ miles on their original battery packs, including an entire fleet of New York and San Francisco taxis. Under normal use they're warrantied to 100k miles or 8 years. It's not something you should have to worry about much for the life of your car.