1 $ for a hit with a hammer, 99 $ for knowing where and how to hit it...

The price usually includes: taking the bike in, doing the paperwork if the owner won't wait on site for the job to be finished (or if it can't be done right away).
Removing the fork from the frame - which requires removing the bars and the front brake.
Cutting it, filing the cut to be clear of any burrs.
Returning the fork, bars and the brake.
Setting up optimal headset bearing preload.
Checking whether it all works (disc brakes with QR wheels often require some fine tuning to avoid any "scratching").

So, not really a 3 minute job.

I'm all for buying the needed tools and doing it yourself, properly.
Like all other DIY stuff: it makes one more skilled, which helps with being more self-reliant, in case of any roadside (emergency) repairs, not having to leave the bike at the shop (when there's peak season rush).

But if doing that for someone else, doing it properly and taking the responsibility in case anything is done wrong, I think 20 $ is a fair price.
For the effort, knowledge, responsibility and the time needed.