If you're already going to spend the amount of money that one costs (say for a higher end gas-only sedan), then yes, you will save money with a Prius or any hybrid.
However, if you're trying to spend extra money for a hybrid in order to save gas money, you will not do so in the time that Americans typically keep their cars (5 years, for my math):
My gas mileage (Ford Focus) on average: 25 mpg
Prius gas mileage approximately: 50 mpg
Cost / 12 gallons of gas in my car, N. California: $26.40.
This takes me appx. 300 miles. In a Prius, you'd go appx. 600 miles.
Difference in cost:
Prius - Focus: $30,000-$15,000: $15,000.
$15,000 / $26.40 = 568 12 gallon fill ups
568 12 gallon fill ups at 600 miles/gallon = 340,800 miles.
Yeouch!
Hope that helps!