Adding miles to a lease is cheap if you shop right: +$20/month for 12K miles, +$30 for 15K, +$35 for 18K. But you must first find the "winning dealer" using the 1.5% rule—divide the monthly payment by the car's total MSRP. If it's over 1.5%, walk away. High-mileage leases only get expensive when you let an "80% dealer" pad the deal with fees.