Bought gas for the first time for the Prius today; 8.898 gallons (US), which means there were about 3 gallons left even though it beeped and started flashing the last square on the fuel gauge. 384 miles on the odometer, although it's not entirely clear how many of the first 14 were on gas already in there, so I suppose the 43.2 mpg isn't perfectly accurate. Close enough, although removing those 14 miles drops it to 41.6 mpg. Whatever.
One thing that's interesting about the Prius is that the tank has a flexible bladder in it, so the capacity can change depending on the outside temperature... I assume it doesn't fluctuate much between 50 and 70 degrees, which is where we spend most of our time here in the Bay Area.
At 2.399 (prices are down) per gallon, I spent $21.35 for 8.898 gallons of gas, or 5.56 cents per mile. Assuming 12 miles per gallon, which is what I was getting in the Pathfinder, I'd have needed 32 gallons to go the same distance. 32 gallons would have cost $76.77, or $55.42 more.
The greenhybrid link for our car is: http://www.greenhybrid.com/compare/mileage/car/1564.html.