What do most cabbies drive? Usually a Crown Vic or a minivan around here. A Crown Vic is no $20k car, neither is a minivan. They run similar or higher pricetags in comparison to a Prius. And if you follow the number of miles a cab can accrue in a busy market like San Fran or NYC, getting better than double the mileage will give a very fast ROI. 50 mpg, is indeed 50 mpg. If a person is looking at $30k vehicles, they're not likely to go and look at a 15 yr old VW TDI. My uncle spent somewhere right around 18-22 or so for his Prius, like I said, he bought it used. Driving Phoenix, Az to 29 Palms, CA on one tank, he averaged 50 mpg. That included navigating Phoenix traffic (which sucks), most likely getting caught in a traffic jam on the 10 through Phoenix, and then the highway driving itself.
As far as what numbers are claimed for a Prius, the last published numbers I saw were 54 city, 48 highway, +/- 1 or 2 mpg. A cab will see the same city mileage as any other driver, in a Prius at least. When not moving, the engine shuts down except to run the A/C or stereo if the batteries are dead. In my uncle's we got all the way to 40ish before the engine kicked in while being easy on the gas pedal.
EDIT: Looking up the actual numbers from the EPA (which are the numbers posted on the cars and advertised by the mfrs), you chose to quote the numbers from the Prius V, their new minivanish variant. The stardard Prius is still rated at 51/48, with the compact model at 53/46. Also, the high end MSRP on all three is still under 30k. Comparing them to the most common cab, the Crown Vic, city mileage is better than 3x what a Crown Vic gets while the Vic's MSRP starts where the Prius tops out. If you want to debate easily verified info, at least try harder to distort it...