In my case, the choice was keeping an Acura TL or buying a Prius.
Right now, at $4.00 a gallon (which actually is low), and driving 12,000 miles a year, using actual mileage for my cars:
Acura: 25 MPG means 480 gallons at $4.20 a gallon (premium) is $2,016 a year for gas.
Prius: 48 MPG means 250 gallons at $4.00 a gallon is $1,000 a year for gas.