Thirty percent of the people who got new cars last year decided to lease instead of buy. The good news: Lease payments are lower than auto-loan payments, so you can drive a car for less. The bad news: Consumers are suffering from sticker shock as the average vehicle price rose to $21,000 in 1996 from $18,000 in 1994, just two years before.

When judging a lease, shoppers usually look only at the monthly payment. That works if you're sure you will keep the lease for its full term and if, at the end of the lease, you will turn in the car and lease a new one. Given two comparable leases for the same car, the one with the lower monthly cost is best. However, if you plan to buy the car when the lease is up or if you're forced to break the lease early, that strategy may not work.

At that point, you need more information from your dealer. Typically, though, the majority of lease contracts fail to disclose all your costs. Auto-lease disclosure is governed by the 18-year-old Consumer Leasing Act. The Federal Reserve Board has published revisions in the act's regulation, which should shed more light on a lease's costs.

To calculate leasing costs, a great resource is the Reality Checklist, a 17-question work sheet created by Ralph Nader's new Consumer Task Force for Automotive Issues. For further information, you can get it free on the Internet (the address is gopher.essential.org) or for $1.50 plus a self-addressed stamped envelope to Reality Checklist, P.O. Box 7648, Atlanta GA. 30357.

The leasing process begins when you pick out a car at a dealer's showroom. First, strike a bargain on the car price. For example, you might get a $25,000 car down to $22,500. When the lease is up, you can give back the car or buy it at a predetermined price, say $15,000. Your monthly payments are based on the $7,500 difference between the car's upfront cost and is end-of-lease price, plus interest and incidental expenses. In a sense, you are financing the depreciation cost of the car. A few key points to watch out for:

• Do not ask the dealer, "How much a month?" without negotiating the car's basic price. The dealer might then base the payments on the full sticker price or higher.
• Even if you negotiated a lower price for the car, the dealer could use a higher price to calculate your monthly payments - and you wouldn't know.
• You might intend to buy a car outright, and bargain down the sticker price. The dealer may then offer you a "special low-cost deal." You sign without knowing it's a lease or that the car price is higher than was agreed.
• The dealer might swipe your trade-in. He offers, say, $3,500 for your old car - but that full sum is never subtracted from the price of the new car you lease. The trade-in price is supposed to show on the lease, but not all dealers put it there. Ask to see the calculations.

To defeat these schemes, you need to know the true price of the car that you're going to lease. The term is referred to as the "capitalized cost," and should include incidentals such as fees, taxes and rustproofing. The Reality Checklist shows the cap cost both before and after the down payment. The American Financial Services Association, which represents several large leasing companies, defines cap cost as the car's price after the down payment. This cost should be disclosed to you, the consumer.

To minimize your costs in case you have to break your lease early, you want a low monthly payment and a low cap cost. If you intend to buy the car at the end of the lease, add up all the monthly payments plus the end-of-lease purchase price. This is known as an open-end lease and is not recommended. The car with the lower total cost is the better deal. Remember: It pays to comparison shop!