You can't come to an agreement, you can sue the inspecting shop or business for the cost to repair; in that lawsuit, you can provide evidence of the cost to repair (though will need more than one quote; I have never seen a judge accept just one quote as being convincing evidence), while they can provide their evidence of a lower cost; the court will decide how much is proper. Of course, suing takes time and costs money; it may be a good idea to accept less money than you'd ideally like in order to avoid a lawsuit.