We just had a good, long-time client email us to ask if the tech really makes $x per hour (x is our service rate).

How do you even respond to this?

Obviously if we only charged for the tech's rate, we would lose money. Who would pay for the truck, gas, insurance, etc?

I'd like to respond in a diplomatic way, but I'm kind of dumbfounded right now and not thinking straight.

I'm hoping someone can come up with a good response as I said the client is one of our oldest and best, so I'm not sure where this is coming from.