Generally, hybrid cars cost more upfront than their conventional gasoline counterparts. For example, the 2015 Honda Civic Hybrid lists for [x] but the non-hybrid 2015 Honda Civic sells for [y]. Of course, the hybrid gets better gas mileage, so the added cost should pay for itself over time. However, the success of this extra investment is in no small part a function of how much you drive and the price of gasoline.
Identify a car that has a hybrid counterpart, and build a model that makes a recommendation as to which to buy based on an assumed number of miles driven per year, a flexible number of years of ownership, and an expected cost of gasoline.