Second Party over billing (*********** people that are not their customers)

Late Last year my car rolled onto an outsourance customer which created a dent on their bumper , showing that the impact was not of great nature my car just has scratches. Outsourance contacts me and tells me they have trusted dealers they use to fix and repair their customer cars and fixing the dent of a 2014 model etios will cost me R15 899. I consulted with other dealers and the price I was given by more than three of them said it would cost less than R4000. The insurance then contacts me and tells me I have to pay the whole amount or they taking me to court and now their clients bumper costs R17 629,17 . I have no idea if their clients cars are made out of gold but this is ripping people off.