The following link is a calculator for some popular Plug-ins. If you're thinking of buying a plug-in, the first thing you should do is don't believe when people say "You'll save a lot of money not have to buy gas." How much you'll save greatly depends on how frequent you could charge your battery. A lot of people could only charge once a day at home. The battery capacity also goes down during cold months. Before tax credit, the premium for a PiP is $5000 over a standard Prius. If you're not qualify for the tax credit, you may never save enough to recover the higher cost. On the other hand, if you're not interested in saving money and just want a car that could move in EV mode, then the PiP is a good fit. It's less expensive than other pure EV's and you don't have to worry about the range anxiety. PHEV Fuel Economy
That's not entirely correct. The PIP suffers from the lowest federal tax incentive ($2,500 vs. $7,500) and limited manufacturer/dealer incentives. Currently, the Volt, CMAX Energi and Leaf are cheaper than the PIP or close enough that price shouldn't be an issue. If you do not qualify for the max federal incentive and do not wish to lease then the PIP may be a better choice. I agree with the rest of your post though. Depending on the price you pay for a kWh and the distance you travel in EV mode the cost of a plug-in can actually be higher than a regular Prius.