How long do they last? I wouldn't know because Toyota and Honda claimed they haven't had to replace one yet, they estimate 150,000 miles or roughly 240,000kms and the warranty is for 8 years or 100,000 miles. Toyota apparently has said the replacement costs of the battery is roughly $3000.
9 years and yes it is very pricey. Ive seen several Prius battery's done at my dealership and there's a lot of shit to be removed to get it out. Takes roughly a day from start to finnish
GM is requiring a 10 year minimum on it's battery packs for the Chevy Volt. The company who is working on them claims 7000 cycles (charges) out of the new Li-Ion design which they say should do the job.
One thing to point out is many laptop batteries right now only handle right about 300 cycles, and seem to have an average of 3-4 years.