A better way of thinking of it is to compute the break-even time. Consider the Prius vs. Matrix comparison in the study. Even if you accept the 20,000km (12,000 mi) assumption and and the 1.17$/liter gas price, the difference in fuel cost accumulates year over year. According to the EPA, the Matrix has an average efficiency of roughly 30mpg (12.75km/liter) whereas the Prius is around 50mpg (20.28km/liter). Using the numbers in the study the Matrix annual fuel cost is 1.17 $/liter * 20,000 km / (12.75 km/liter) = $1835. The same calculation for the Prius gives $1154. This is a difference of $681. According to the study the 5 year difference in cost was only $1718. Therefore based only on the gas cost break even would be another 1718$/ 681$/year = 2.5 years.
In other words the break even time, using the numbers in the comparison, is roughly 7.5 years. The break even time drops considerably if you drive more than 20,000km per year. Add to this that the Prius is a much nicer car than the Matrix and you get the conclusion that if you keep your cars for a reasonably period of time, the Prius is MORE economical, and a better value.
"When the going gets tough, the tough get empirical." -- Jon Carroll