I don't sit down and decide "here's how much I'm going to pay the plumber." I call a plumber and get a quote. I can keep going until it feels worth it.
My enlightenment comes from talking to a bunch of plumbers and hearing what they're gonna charge. That's the market rate.
If I hire an employee and they get good at their job and someone else offers them a 40% raise, they'll probably leave. That's a lot of money.
Some companies care about retention, most don't really.
You're making the wrong comparison. If your plumber increases his rate by 40%, will you "fire" him and get another plumber which will also be 40% more expensive, or a plumber which costs the old price but isn't good, or accept his price increase?