Comparing NOS prices for a concours restoration to repro is a bit of a stretch. Apples and oranges.
1 person on a forum doesn't show much demand, or 10... or even 100. And of the 20,000 convertible owners how many would really pay up the $275 for the set... very, very few. And then to make them "concours correct" would require even more care to satisfy the demand of the high end resorer as the example set does show some machining marks not on the originals. People would care.... more cost. And the fact one owner made 10 sets as one-offs out of love for the hobby and has the CNC files to make more does not show "demand" from a business case view.
And on the cost issue, it is a factor but the examples of high dollar parts for a handful of cars does not show commercial quantity demand. Or someone like Drake would have stepped in and expanded their business.
Ya can't even get a decent carpet or bumper, so whats the hope for some odd lower demand part at a "reasonable" price the market will bear... Look at CoralSnake's site, fantastic repro parts, with prices to match. Same for the shop the makes the exact repro exhaust systems (forget the name right now). Incredible workmanship, but IIRC over $2k for a setup, not many are going to go there. Ii won't. Some will.
Prototyping from 3D printers is dropping that cost, but tooling and the actual run are the big cost. $25k can go real fast. A few years ago I was quoted $10k (in Australia, expensive place) for tooling and a test run for two very simple bushings I did in 3D CAD I was looking to make a while ago. Production runs required 5000 minimum and at 20k started dropping per unit costs.. but there was no way I was going to drop 30-40k in 10,000 $5 parts (10k "profit") and hope for a return dribbling in over 10 years... that's charity, not business.
Do the math, make the run if you're so into the production process, sure it's such a money spinner, and that cost is not an issue to every Mustang owner who wants a widget!