You'll also want to see where in the transaction the leasing company is giving you credit for your trade-in. Remember, it's just as if you gave them cash for the value of your old car. Make sure that they're not giving your credit at the end of the lease or in some complicated formula. Not all cars are created equal here. You want a car that will hold it's value well during the lease period.
Some cars depreciate more than others in the first few years.
Suppose you're about to lease a mini-van. There are two easy ways to do a comparison. Take the wholesale value of a three year old Voyager and divide it by the cost of a similarly equipped new one. Now do the same for a Chevy Lumina. The ratio you've calculated is the percent of purchase price that the car maintains after three years.