I'm a (16 years) retired Machinist, Tool & Die Maker, CAD/CAM Operator, CNC Programmer/Operator, CMM Operator so I have an idea of what's involved in the OP's task and can somewhat sympathize with him.
The first few years of my retirement I was allowed to go back and use the machines for farm equipment, gun, and motorcycle machining projects but policy changed and that came to an end. I actually toyed with the idea of buying a mill and lathe to handle my farm equipment, gun, and motorcycle machining needs and even went to a few auctions but reality set in and I just couldn't justify the expense for no more than I would actually use them.
I do have friends who own machine shops so if I really needed anything I could avail myself of their equipment.
It is somewhat distressing to get these quotes when we know we can do it ourselves but as has been mentioned several times those shops are in BUSINESS to earn a profit. While $90 may seem stiff to those of us who have the skills to do such a simple task the reality is that it's no doubt worth every penny to a business to take on such a small simple job.
It was mentioned about installing adjustable sights on a fixed sight handgun.
Back when I was working a local gun store asked me to do some machine work on 1911s for his customers that he wasn't able to do.
I made a fixture for the slide and bought all the (carbide) endmills, drills, taps, and dovetail cutters to do BoMar rears and Chip McCormich fronts.
It was a time consuming and very stressful task.
I did a number of them for him and don't recall how much I charged but knowing me it wasn't near enough.
I did the sights on my Stainless Delta Elite as well as other modifications and it was my EDC for years.
I guess to me the bottom line to me is that quality machine work is worth every penny if you don't have the skills and resources to do it yourself.
On an aside B-Square used to make a base for the older S&W revolvers that used the front sight screw and locked into the rear notch. I've used one with a 4X scope for checking loads on everything from a 617 to a 629 with no problems. On one of my 625s that had the three screws drilled I also have a B-Square for it.
James