I rarely get push back when I give an estimate. Probably because I think my work is very fairly priced. I've learned to do a bunch of things because I wasn't willing to pay someone the going rate, so I'm keenly aware of sticker shock.
The few times I've been questioned, I've always had a set response, and that is: "What would it cost you to do it yourself?" , and, "Do you have the knowledge/tools to do it yourself?" Oftentimes, that hasn't been thoroughly thought out by the customer.
When I worked in the bicycle industry, a customer came in wanting me to install a headset. A headset is what holds the handlebars, stem, and fork together and allows it to turn. It is a pretty simple set of "cups", spacers, and bearings. I have probably installed a couple hundred headsets in my life, and can press an old one out, press in the new cups, assemble it, and adjust it in about 10 minutes. I quoted the guy the requisite $30 charge, he reluctantly agreed, and I went about my work. When I was done, he said "That was easy, why should I pay $30 for THAT?". I gently told him that I had used about $400 worth of tools during the installation, that I was very efficient, and that I had the knowledge to install it perfectly every time. He still pressed me, and I told him that he could do it next time, and I would cheerfully sell him the tools and he could buy a book (that we also sold) explaining how to do it. He wasn't too jazzed about doing it himself after that.
My line of work requires a lot of knowledge; some simple, some not so simple. It also requires a lot of tools...thousands and thousands of dollars of tools! I advertise, I put gas in my truck, I carry liability insurance, and I spend a lot of time learning new techniques and acquiring new skills. All that is factored into my estimates. If someone needs a project that takes only a couple of hours, but requires a lot of tools or intricate detail, it will be reflected in the price. Even if I'm just putting in some base trim, that requires a lot of tools the common person might not own: A miter saw, compressor, nail gun, caulk gun, stud finder, pry bar, coping saw, and a tape measure (at a minimum). It's a fairly simple job, but I use over $1,000 in tools...tools that wear out or need maintenance/parts, regularly.
My job is not unique in these facts. An good auto mechanic probably has $20,000 in diagnostic tools alone. Yes, these tools are used over and over, but they require a huge investment, sometimes a lot of education to use them, and maintenance. However, I've heard folks complain that they had to pay $45 for an oil change when they pay a maid $100 to clean their house in 2 hours.
The next time you buy something or hire someone to do a service, think about all that person has/had to do to perform their task. If you consider it all, and still have your doubts as to their value, ask yourself if you could do it yourself for less.