I'm also trying to do the math on purchasing a Prius. Although I would love the space of the Prius V, I agree with everyone else here that there aren't enough used ones on the market yet. The regular Prius + roof rack sounds like a better option to me.
In my case, I'm about to start a new job that has me going from a 6 mile (one way) bike commute to a 30 mile (one way) highway commute. Current car (1977 Mercedes 240Diesel ~107000 miles) gets 28ish mpg, trying to figure out if I can justify the cost of a 2005-2009 Prius in gas savings over a few years. Looks on craigslist like you can get one in pretty good shape for under $9000.
The only real problem that I've found with the used prius market, at least here in southern california, is that many of these cars are used as heavily driven commuter vehicles, so it isn't uncommon to see a 2007 prius with over 250,000 miles on it. No idea what their mileage life is, but this seems like something to be avoided. Anyone out there with a realistic recommendation on the max mileage you should purchase a prius with? I'm thinking no more than 150,000, but that's a number I just pulled from nowhere.