The cost of machining plummets when gunmakers start out with castings made in the shape roughly approximating the finished part. By contrast, "old school" P210 frames started out as forging that weighed 2.33 kg, ending up at .033 kg after 107 machining operations.
Both forging and casting result in blanks that are approximately the same shape as the finished part and both forged and cast blanks require serious hogging out in order to obtain their final shapes. That's part of the entire point of forging, besides work hardening the material. The P210 frame is not machined from a billet like the "flame pantographed" Inglis High-Power (made by a washing machine company, of all things) or even the original AK-47 receivers. And for some perspective, until CNC became king, it took over 2000 operations to build a simple S&W Model 10 in the 1950s, not including 500 different inspections. Recall that, until the late 1990s, nearly every part of a S&W Model 10 was forged, machined and hand-fitted.
The word from P210 historians is that SIG found the inverted frame rails much harder and costlier to machine to the required dimensions and tolerances. I have no idea why the same considerations should not apply to the slide rails on the M1911. But absent reliable evidence or knock-down arguments to the contrary, I am inclined to take them at their word.
I am not sure who these historians are, or whether they also happen to be engineers as well as historians, so it is difficult to respond to this argument. However, this does not change the fact that the Czechs, Italians, Israelis, Turks, and Chinese are also somehow capable of machining inverted rails in CZs and CZ clones without resulting in one, two, or three thousand dollar guns.
Again, I do not say this to diminish the quality of the P210. There is no question that it is a fine piece. All I am trying to convey is something you say yourself:
No other postwar service handgun was nearly as labor-intensive.
This is the key to the high cost of the P210: labor, not design. The gun was expensive to make because it was largely made by hand by Swiss laborers. Cost, volume, and production efficiency were not priorities, so SiG could indulge in more time-honored methods of manufacture. For comparison, look at the Smith and Wesson Registered Magnum, considered by some to be one of the finest revolvers ever made. From a design standpoint, it is essentially the same as any other Hand-Ejector. However, each RM involved a tremendous amount of hand fitting and finishing. That is where the cost came from, not a design handed down from god or reversed engineered from alien spacecraft or made from dead unicorns.