Some answers depends on the customers history to how I answer, but you can quote your average cost with profit is so much money, as example $80.00.  You explain to the customer if you don't charge them, then you have to add the free service cost to other prospects and customers, ask them then is that fair to others.   You can also explain that in many industries it cost between $100.00 and $150.00 just to show up at someone's home.   Then charges are added on that for the work.  Driving 100 miles is very expensive with the labor and the cost of your equipment and running the balance of staff location etc.  I believe if you let the person understand you are doing your best to keep prices down and ask how they would handle all of the expenses.   This makes them more aware of your policy and concerns.  You can then better cement the relationship by knowing their concerns  and responding correctly, sometimes in more depth with some customers than others.