Paying less than half the qoutation

My car was diagnosed with having had issues with cylinder head, thermostat packed up and therefore overheating. The RMI approved mechanic quoted R22185 for this and Hollard will only approved to pay for the Overheating which was R3500 and after i disputed this agreed to pay for the Thermostat aswell which is R2500 and not for the cylinder head and engine which is the major part of the quote. I have been a loyal customer and this is my first claim ever since i have been with them. It is unfair that i have had to continuously call them and now have to pay more than half the quotation myself. WHAT IS THE POINT OF HAVING INSURANCE?