Given: MPM (miles per month), $PG (dollars per gallon), MPG1 (previous miles per gallon), MPG2 (new miles per gallon)
Find: SAV (savings in $ per month)
SAV = MPM x $PG x (1 / MPG1 - 1 / MPG2)
If you have an HP17BII calculator, you can enter the equation as written in "SOLVE" mode and the 5 quantities will label 5 top-row keys. Then you can input any 4 quantities (by pressing the numbers and then the labeled key for each) and pressing the 5th labeled key will display the calculated value.
Sample Calculation: MPM = 1400, $PG = 3, MPG1 = 15, MPG2 = 55
SAV = 203.64
So this says the SUV I drive 30 miles to work and back 5 days a week, and average 50 miles on weekends costs me $203.64 a month more than it would if I drove a Prius. In a year the Prius would save me $2443.64 on my gas bill. If the trade-in on my SUV is $15,000, I would break even in 4.09 years. If the price of gas goes up, I would break even sooner - in fact if over the next few years the price of gas averages $4/gal, the break-even time would be 3.07 years. This does not account for any variation in trade-in value at different car dealerships so I assume Toyota allows the same as any other make, dropping it out of the calculation.
If you think rewarding Toyota the $10,000 difference for their development of this hybrid system is better than paying it to Shell Oil and adding to the world's pollution for 4 years, it should be an easy choice.