For a long time, you had to purchase a hybrid out of the goodness of your heart. In other words, for the longest time, the likelihood of your owning your hybrid long enough for it to payoff was very, very low. You really needed to go hybrid for non-economical reasons (like your love for the environment).
The good news is that many hybrids are now cost-effective. They pay for themselves in less than 5 years, under normal driving conditions. Of course, you should keep in mind that hybrids are much more effective at saving gas in city driving as opposed to highway driving. If you do mostly highway driving, your hybrid purchase might still need to be an act of conscience, and not an attempt to save the piggybank.