I agree. These prices are completely crazy, especially if the fit requires a lot of work from a bodyshop.
Consider this: back when these cars were in regular production, factories were pooping them out the doors faster than can say wow, and they had proper fit. What can possibly so hard about making parts that fit right. That's beyond my understanding...
Don't know if the price is crazy. It's probably more expensive than you think to make a whole body. Making perfect body parts is very expensive. Ford could afford expensive dies, because they made ~500,000 of each part (in case of early Mustangs, at least). They could spend a million dollar on a die for a driver's side Mustang door and it would only cost them a dollar per car. And besides, factory body panel fit wasn't all that impressive (although I have to admit that, considering my age, all I've seen in 30+ year old cars, so who knows what happened to them in 3 decades)
That doesn't automatically mean that the Dynacorn body makes economic sense for a Mustang restorer though. One of the things is that you always need more parts than you think. Even if a car needs "everything", there are still tons of small, usuable parts on a project car. Steering columns, seat frames, pedals, window mechanisms, glass, stainless trim etc. Wether this is of value or not depends of course on how much you intend to modify the car but all these small parts add up quickly when you need to buy each and every one of them (like with the Dynacorn body). This is of course a non issue if you want to rebody a rusty car you already have.
I don't know anything about the quality of the Dynocorn bodies, but A pillar modifications to make the windshield fit sounds scary. But how knows, maybe Dynacorn is eager to make a good product and they manage to solve the issues quickly.
As for the reinforcements; they are nice, but they're commonly available convertible parts and adding them to an old body is not too difficult (easier than rust repair).
I guess these Dynacorn bodies are mainly interesting for high end restomods where the cost of the body (or project car) is only a rather small portion of the total cost.