Hey Ciderbarrel,
From my experiences most people rarely keep there car more than 5 years these days here in the US. So you would most definitely be an exception! I also know that the vast majority of car manufacturers base there paybacks on a 12,000 mile per year driving average.
Based on these figures the majority of people are always upside down or in a negative equity position when they go to trade in for a new car. Which is how the car dealerships make a big portion of there money.
Based on your driving and figures it definitely makes more sense to go used instead of buying new. If you can get a used Prius for 12K then that makes a lot of sense for you and the majority of people. For people that don't put much miles on the car per year and keep there vehicles long term buying a new hybrid for around 16-17K would be just about right I think. The problem is the cheapest is over 19K which would turn you to the used market.
The sweat spot seems to be just under 19K for most people similar to your revised figures
