To figure out how much you can save, you can use this fuel calculator to get a rough idea of how much you can save by driving a hybrid car. With this calculator, you can compare the saving between a hybrid and a conventional car in terms of current gasoline price per mile. With this figure, you can estimate how many miles you need to drive your hybrid to breakeven its higher retail price than a conventional car.
Because government is advocating people to use hybrid car to reduce the greenhouse effect, to get a more accurate the saving of your hybrid you should take this into account as well. To check out the tax credits provided for each different model, you can refer to this reference page to figure out the tax credit you can receive for the model you would like to purchase. Among different models, 2005-08 Prius seems to be the one with the most tax credits. With these information you can now roughly estimate whether it is a good deal or not for you to buy a hybrid car.