Anybody who has been in business for a while successfully will tell you that if you try to make money on every single job, you will end up failing.
In my end of the world, that is true of small business. If you have a monopolistic hold on the market and your customer by the neck you can make them pay every time.
Small businesses in any line have to respond to the customer on price and service because they need their customers and can't tell all of them to take a hike, although there are days..... The Amazons and Allen-Bradleys of the world have billions in cash, so no one customer, even a big one, matters much to them.
I am fortunate in my trade that my customers know they are over their head and glad to have found me. Hitch's customers think (like many many people who want to own a restaurant), heck I know how to write (cook), how hard can it be to make it an ebook (run a restaurant.)
I think both are hard rackets when the customer thinks they can do it themselves, yet know very little. What is the phrase, know just enough to get into trouble?