I know what you're talking about. I ran into a guy that had a plug in conversion on his prius. He said he goes 60 miles on just battery power before the gasoline engine turns on. The conversation was brief but he said he paid $9000 for the conversion, which was new software and battery with the plug in outlet. It was very interesting but I think I'll wait for the conversion from toyota in a few years.
Then you have to pay for electricity and replace batteries?
No, I think he said it was an add-on battery to the existing oem battery. I did some research on the kit and depending on what you pay for electricity it would cost 75 cents for a full charge, which usually takes 4 hours. Still 9K is ridiculous for just a battery and software. I assume it will get cheaper in a few years but by that time the plug in prius should hit the usa I hope.