How often has a customer asked you for an estimate or a 'ball-park' quote before the cause of the problem has been properly diagnosed or inspected?
And, how often have you given an approximate price only to find that the final repair cost is not even close to what you originally suggested.
Stop doing that unless it's a straight forward maintenance service that you definitely know what the final price will be.
You do not know the price of the parts without doing some research and you certainly don't know the labor costs on that particular model/engine combination.
If you do give a price ahead of time, even if you say that it's a 'ball-park' price, the customer will hold you to it and you will be left eating the balance leaving a bad taste in your mouth.
Let them know that you do not give estimates without doing a little research so you can give them an accurate estimate of the repairs to be done.
I know you are trying to be the 'good guy' in answering all their questions, but in doing so, you can become the 'bad guy' because the price was higher than you quoted. And, that's not a good thing.