You were at fault in damaging their car by definition, if you hit a stationary object  a wall, you were driving negligently, or carelessly, and so would be at  fault. That obligates you to pay the actual cost of the damage, or such lesser amount as you and they agree to. Bear in mind that if they feel the cost to repair is more than you are willing to pay, they could sue you for the extra money, and if they could prove the higher cost, would recover it from you. It is in your interest, based on the facts you describe, to negotiate something mutually agreeable to avoid the time and cost of a lawsuit. Remember your liability is a given, since you backed into a stone wall the only issue is the cost to repair. You may wish to get several more repair quotes, to see the true cost of a repair.