Depends on what you mean by "extra" cost. If you are not planning to buy a new car, buying one just to get a hybrid means the entire cost is extra. If your planning to buy a similar vehicle, the "extra" cost is actually pretty minimal. Especially when you factor in tax incentives for the year that you buy the car. If you drive two thousand miles a month the comparison to a brand new Honda Civic will leave you with a little more than an extra $136 dollars a month. About $1600 a year. As mentioned above the batteries do wear out, so when you've saved more than $13,000, you'll have to do some spendy repair work. Keep in mind that those batteries, like any technology, will be a fraction of the cost eight years down the road. Not to mention that few people keep a car that long.
Now, like I mentioned, there is the tax incentives. The federal government gives you a credit for the purchase of a new hybrid vehicle. Starting next year, and for two years, WA state will not be charging sales tax on them. You've managed to save several thousand dollars more there.

Now, realistically, you're not going to be lining your pockets with cash by buying a hybrid at a savings of about 6.8¢ a mile. But, to specifically answer your question, decide how much, if any, more you're paying for that car than your next choice and divide the price difference by .0068. Realistically, I think you'll be surprised by how little the difference is to buy them in the first place.


(My figures are based on 2008 Honda Civic non-hybrid getting 30 mpg, yes, I know some get more. And on 2008 Toyota Prius getting 55 mpg, yes, I know some get less)

Just a side note. The Prius and other hybrid car owners I've met have, without exception, been very happy with their vehicles.

The thing about hybrid is this. Yes you save a TON of money on gas, however I think every 8 years or so you have to replace the battery, which is thousands of dollars. So you're savings are almost completely canceled out. It is better for the environment though.

Miles? In an abc news local broadcast from Kansas City MO they offered a link to calculate that and a $25,000 was going to take 151 months to recoupe the gas at $4.00 a galon. Now by miles ? They offered a calculator for the car you wanted to buy. Here is one for example:
http://www.edmunds.com/advice/fueleconomy/articles/116513/article.html

$4 a gallon is on the low side. But the real flaw here is the $25,000. That's only if you buy one without needing to buy a car, and without trading in your old one. How many people fit such a description?

there arent enough miles if your upside down in your trade !

There are a lot of variables such as how much one drives, and what type of traffic you are normally driving in that affect any chance of payback on the current hybrids. Even with higher gas prices, the balance isn't nearly what one thinks it might/should be.