Hybrids may or may not save you money it depends on a lot of different things; miles you drive on average, gas prices, type of driving you do, what you're comparing it against, etc.
You're leaving a lot of important details from your math though, like the fact the prius is still worth way more at the end of that time than the accord would be. Cost per mile is hardly a new concept but it takes in to consideration a lot more than mpg. There's maintenance costs, depreciation, reliability, and much more.
If you want a way to check it, edmunds has a great little tool that will break it down for you (for new cars over 5 years).
http://www.edmunds.com/apps/cto/CTOintroController