While there is a time-value factor, i dont think its that clear cut. For a few dollars, you'll have the materials on hand and the experience to mod your case as you see fit whenever you feel like it. It just opens up options.You're missing two key calculations here:
1) Time - at 20$ An hour if it takes you more than 10 minutes to make one, you're losing dramatically.
2) Ease of access - I'll absolutely pay the extra for something that is a drop in place solution.
Now if it were a 20-30$ filter you could make for 3$ in material? Then, go for it, the time investment is worth it.
So for the power cable comparison:
People buy pre-made power cables because removing pins from those ATX style connectors is a pain in the fucking ass and not worth the time and effort for most of us. (I've done it, but if I had to do more than a few, I am absolutely buying a pre-made cable.)
Power cables, those connectors are like 40 cents a pop, and free if you have an old PSU you are replacing(i kept new ones as backup). I bought from Ensourced, fyi. Bought a huge roll of wire off ebay for a few dollars. Crimper for $25 from Amazon. I dunno, it only took me a few minutes to create a new set power cables for my new GPU, same for multiple times i changed drive layouts and when i realized a cleaner way to route CPU power(instead of near shortest path). While first time took a bit of time to wire up everything brand new and also learn how to crimp/double crimp, size wire right... Now its nothing.
I dont play baby dress up/action figure, no fancy heat shrink/sleeves,... The expensive/overrated stuff is when you go all peacock show, i fully agree with you there. But there is a lot of doors opened if you invest a few dollars on a crimper here or dremel there... Its not expensive at all. Like i showed, $20 will likely get you enough material to last you many cases and or re-designs. And the 3rd time on, it will take a few minutes at most.