Using 13600 miles per year, and comparing against a baseline average of 19 mpg (numbers extracted from previous referenced article), I assumed you'd get 48 mpg in the Prius, which you would buy for $21K. This suggested it would take 11ish years to pay for the Prius in gas savings over the 19 mpg car. If you were able, like R., to get 44 in the Fit which you bought for $16K, by comparison, it would pay for itself in gas savings in 9ish years. But you probably aren't R., and if you are, you'd probably do better than 48 in the Prius. (OTOH, you might not be driving 13600/year either.)
If you get 60 avg in the Prius, it pays for itself in 10 years.
What if gas continues to go up?
Back at 48 mpg in the Prius, at $6/gallon, it pays for itself in gas savings alone in 8 years.
At $10/gallon (when the world has probably ended, and definitely you've gotten the bike out of storage and have more people living in your house, either a renter, or a down-on-their-luck friend/relative), the Prius pays its entire purchase price, in gas savings over the your previous vehicle which got 19 mpg, in under five years. You may, at this point, be running a taxi service for all the people who can't afford to drive their own cars. (The Fit, if you are R. and get 44 mpg, paid for itself in 4 years.) (If you got 60 mpg avg out of your Prius that whole time, you paid it off in 4 years and 3ish months.)
Expecting your next car to pay for itself _entirely_ in gas savings over your previous car is loopy on several levels. After all, people do have to periodically replace their cars, either in the form of increasing repairs or buying a new (to them) car. Maybe a different way of looking at this problem is called for.
How much is someone in an average car running an average number of miles paying for gas over the course of the year? with gas at $4.30, over $3000/year (and people were complaining at the high cost of gas when it was _half_ that). People finance cars. If you finance gas (credit car, anyone?), it really gets crazy fast (bankruptcy court, hello! Wait, what do you _mean_ you changed the laws?). Running a Prius (at 48 mpg) costs you a shade over $1200 a year in gas. That's like paying $1.80ish in the 19 mpg car.
This seems like a _much_ better explanation for why people by Priuses (Prii?). (The Fit for R., a little over $1300/year.)
People like the woman I referred to in the earlier article, who just bought an SUV a yearish ago, are in a car-financing place where swapping requires a lot of justification. People who are already needing to buy a new car, really, really easy decision to go with the higher mpg car, even if it costs a little more. (And for the record, the Versa, Yaris, etc. are all cheaper than the Fit, altho I don't know what R. could get out of them in terms of mileage.) In the meantime, carpooling, taking readily available public transportation, reducing trips, etc. is easier and cheaper yet (the less you drive, the longer it takes for a high mileage car to justify its existence. Of course, it _lasts_ longer, then, too.).
I'm still trying to figure out how to run numbers of moving from an outer burb to an inner burb. Very tough to find valid simplifying assumptions. It's also worth noting that as soon as you insert calculations about trade-in value, financing costs, etc. in car calculations, you can make these numbers do any damn thing you want them to.