Disgruntled with the disservice service and Costing

I have been disappointed with the service rendered by Kia Multifranchise Stikland on the 09/01/2019. Initially, I had sent my vehicle to them on the 12th December 2018 when it had the engine light problem and loss of power, Kia Sorento 2012 model, however they were fully booked. This was based on the previous excellent experience with engine servicing at regular intervals. I went again on the 09/01/2019 and explain to them what the problem was as I had done my research of the problem, faulty EGR-valve. I was told by the workshop supervisor/ foreman that they will run diagnostic tests, though I had told him what the problem was. I got charged R690.00 for the problem I had informed them of. All he needed to do was to drive the vehicle to establish what I had volunteered as a diagnosis of the problem was indeed true. Then the cost to repair the problem was set at R8080.00. I refused as I saw that I was being reaped off and was able to fix my vehicle at a cost 51.7% less to what they had quoted me on.