That is exactly what they are doing...
If you are charging £100 for something and it only takes you an hour then they are wondering about how much you are earning.
I wouldn't let this put you off though... how often have you taken your car in for a service or repair and it has cost you a couple of hundred quid... it might have only taken them 10 minutes, but that is the going rate so....
It is best not to get into a conversation about insurances, taxes etc, but just to say; that is the price, if you would like to think about it and let me know... blah blah blah
Don't feel like you have to justify your prices, as long as you are comparable to those local to you then you should have no problems
Andy