I would put 100k-150k for a lifetime of a car, most people don't hold on to them even that long. In the Prius case that's not even including the cost of replacing the battery packs which I would expect to wear out in that time frame. So at best you have 10k savings, probably less, which means that keeping an older car and putting more miles on it (taking it past 150k like your cars) is probably more energy efficient than building/buying a new one every couple of years just to take advantage of a 5-15mpg difference.
This is different than computers as they are both getting more energy efficient _and_ more capable leading to consolidation in addition to the per CPU core power savings so it's more like a 20:1 efficiency improvement, actually 40:1 because you usually spend as much in cooling as in power usage. A 20 yr old car only has minor capability differences with a modern, high efficiency car.