ROI is something you have to calculate on your own.

1. Go look at your odometer to get an idea for the number of miles you put on per year.

2. Take your current vehicles MPG to estimate the number of gallons of fuel you have to buy per year.

3. Take the MPG estimate of this car and estimate the number of gallons of fuel you'll need if you drove this.

4. Subtract #3 from #2 to figure out how many gallons you're saving. Multiple that buy the price per gallon at the pumps. That's how much it'll save you per year in fuel alone.

If you put on 15,000 miles per year and your current car gets 20mpg, then you need 15000/20=750 gallons. If this car gets 55, then that's 15000/55=273 gallons. This car would save you 750/273=477 gallons. At $2.65 gallon (which is what I pay around here), that's $1,264 annually in fuel. Assuming you'll keep it for 5 years thats a little over $6,300.

NOW... what you really want to know is WHY someone is selling a Prius when most people are trying to buy one. Is it a lemon? Or did the current owner just trade-up for an '06 Prius? Was it a return on a 3 year lease that's up? (hard to believe a leasee would return it. As a leasee you actually DO have the option (under every lease I have ever seen) to sell the car on your own and just pay the manufacturer the residual value which was listed on your contract when you bought the car and pocket the profit. A Prius, I would have to believe, would ALWAYS sell for a tidy profit now that they're in hot demand.