If you go over the mileage allowance on your lease, you will be charged for each additional mile. The rate per mile will be specified in your lease agreement. For example, if your lease agreement allows 12,000 miles per year and you drive 15,000 miles, you will be charged for 3,000 miles at the rate per mile in your lease agreement.
The cost per mile varies and depends on the finance company or dealership in question.