Saving gas?

Chloe and I are probably going to buy a used Prius (an 05), and before you yell at me about it not being significantly more eco friendly (because of the eco-cost of manufacturing) or ultimately more economically sound (they are expensive) let me just say that I've heard most of the arguments for and against. There is one important variable which is a good tie-breaker though, and thats the HOV sticker. In any case, here are some back of the envelope calculations:

subie: 57.3 mile trip (cant take bay bridge in AM) / 22.8 mpg * 5 days = 12.56 gallons / week * 3.30 (prem.) = 41.40
prius: 50.3 mile trip / 50 mpg * 5 days = 5.03 gallons / week * 3.10 (reg.) = 15.59

for a savings of ~25 a work week.