My Insight replaced a '99 Chevy Malibu that got about 25mpg for my daily commute. I got the Insight in February of 2008. Using the same mileage and price at fillup, I came out with $2240.67 in savings over driving the Malibu. This is for 31058.4 miles driven in the Insight.
As an added bonus (though I won't do the math on this one), my wife drove the Malibu from February 2008 to April 2010. The Malibu replaced 'The Beast' for that time which gets 18-20mpg. Not a large difference in MPG, but savings nonetheless. We sold the Malibu in April, and she's back to driving the Blazer. Good news is, she doesn't do a lot of driving unless I go with her. If I go, I drive, and I tend to get it to ~20-24mpg (now that the Insight has taught me how to drive
).