incorrect billing between staff in service departm

I enquired about the cost of 45000km service, which a lady frm service dept quoted R2200. A week later I called to confirm service booking and enquired again on the price of which another service lady told me R1755. On the morning of car service I enquired again the cost due the amount differing on the telephone, a third consultant shows me an invoice of R1160 approx. During the day the same lady tells me that the invoice amount is incorrect and does not included change of plugs and the correct amount is R1932. By then I had no choice but to agree to them changing the plugs as due so in this service. They could only apologise which is not acceptable. They did not enquire on the car details as they are suppose too for the cost to be confirmed but gave me various amounts. This also occurred previously where I was quoted an amount and when I picked my car I had to pay more in this dealership. They must inform their consultants to ask the clients details of their vehicle and not assume and quote incorrectly impacting on the clients budget.