I still don't get how she is ahead. So she's saved ~$10k in gas in the last four years by swapping. Ignoring maintenance, taxes, etc, I don't know of any new Prius that is only $10k. Sure, it's paid off, so she's paying less at the pump than before, but only if you focus on what is being lost at the pump and forget about the cost to actually purchase and pay off that new vehicle.
Either way, they aren't the focus of what I was saying, as they kept a vehicle for 12 years. So in the end, they are in a newer, nicer, vehicle that will eventually pay itself off by gas alone. I don't see someone who is getting the urge to upgrade after only two years keeping the vehicle long enough to cover the cost solely in gas by upgrading from a 2010 to a 2012 vehicle.
There is a big difference going from a 12mpg to a 46mpg compared to 18 to [18/20/22/25]-ish depending on the car listed. Going from 18 to say the 25mpg, and spending $100 a week on the 18mpg car would be ~$72 per week on the 25mpg car. You'd save ~$1500 a year on gas. At that point it makes sense to get the newer vehicle if you can trade the old one in, buy the new one, and come out ahead after the x years you are going to keep it.
The "you" isn't directed at anyone specifically. Just needed an easy to use pronoun there. By looking at the numbers quickly though, if any car on that list was going to have the best chance of paying itself off it would be the wagon, just going by the list prices on their websites. And, it is a sweet looking car. If I wanted it, and had the money, I would get it.