A Prius V is 4.1 L/100 km in combined cycle and 3.8 L/100 km in city driving. It is unclear what the 2.9 l/100 km refers to (and if it's real, like Toyota who said the Prius plug-in would be 2.10 L/100 km), but it's not double the efficiency.

But doubling the range is not that interesting to me, they could make the tank 4 times bigger and that would be useless for most purposes, and that was just my point. The price seems interesting indeed, but that is the price in China, which would be very different elsewhere I guess.