For me, the way I handle such a question depends an awful lot on the circumstances.
For example, most of my customers provide me with steady work. I have a long standing relationship with these folks, and our communication is excellent. We also have a great deal of trust in each other.
I will keep them informed of things that can affect pricing ... we often work together so that everyone is happy in the end.
Some folks are just honestly curious, never having run a business before. I explain to them the basics ... I make my best guess based upon experience, as to the time, materials, and skill required. I will also explain that the nature of the customer is a factor; some folks get charged more than others. More risk = higher price.
If economy is an issue, I provide ways that they might reduce their expense. For example, I will knock $100 off a service change if the customer drives the ground rod. (It's simply amazing how a half hour driving a rod "explains" why I charge $900 to hang $300 worth of parts!)
On the other hand, I have no patience for the game player. I tell them that my price is my price, and they are free to shop elsewhere. If "Joe" will do it for less, well, hire him. I'm not "Joe," and if you want me, you pay my price.
It's not a matter of having "something to hide." We all have our methods. It's not so important 'how' we get there ... just that we do!
For example, LK and I have compared jobs a few times (we're in different markets, so there's no risk of 'collusion.') We differ greatly in how we arrive at our prices, and we treat material mark-up completely differently. Yet, at the bottom line, he and I come in very close together.
Generally, the less said, the better. "I will do this job for this price" is your offer. Accept or decline as you will. No, we're NOT going to perform an autopsy on the quote.
