Maybe it has to do with total production? Possibly more BC-342/312 were produced and were more available? And there is a big weight and size difference in favor of the BC-348. But the BC-342 did come with a AC power supply and was ready to use without modification.

Anecdotally would also speculate more of the BC-312/342 receivers were dumped on the surplus market being by the end of WW2 they were already obsolete  with new receivers entering military service like the 51J and R-388 products that were needed to support teletype making all that WW2 stuff go away. Oddly the BC-348/ART-13 (ARC-8)combination survived longer in military service seeing use in B-29 and later B-36 along with a bunch of transport and cargo aircraft that maybe not as many were dumped into the surplus market as the obsolete BC-342/312 and that can also account for cost difference.  

 I don’t think technical specifications had anything to do with it.  By 48/49 Hallicrafters and National  were selling receivers that way exceed anything you would get with a BC-348 or 342 so serious Hams were buying things like SX-71 or Nationals HRO -5A/C or 7 with the war surplus going to the bottom end market for entry level Hams and SWL users.  The HRO-5TA1 was up for sale as soon as 1946 for $217, think that would be like buying a Icom 7800 transceiver today. Lot of money but everyone knows at least one or two Hams who have to have the best!

The GRR-5 is more interesting. Think the thing there is the battel that started  early in WW2 between AM and FM for tactical communications on the battlefield. I would think by the end of WW2 the battel was over and FM was clearly the winner, no heterodynes, clean good quality audio and excellent short range communications. But the Army did proceed with developing contracts for things like the AN/GRC-9 for HF/AM tactical communications and later sets like the AN/GRC-19 both systems served well into the sixties. That along with things like Broadcast nets where general orders or otherwise one way communications can take place were perceived as having some value. The British PCR receivers were a good example of that. I somehow see the GRR-5 and PCR in some way related in there use? And like the PCR cannot see the GRR-5 having any real value as a CW receiver or otherwise  but as a network monitoring receiver for things like time information it had its place. GRR-5 receivers were mounted in operations and teletype centers for years beyond any of the other radios like the R-392 and T-195 were in service. Least that’s my speculation, the GRR-5 was part of the great GRC-19 generation, where as the GRC-19 configuration survived a little longer in military service because it was capable of running teletype the GRR-5 had little reason to be around as long.

Least that’s my speculation, let’s see where I am right or wrong.

Ray F/KA3EKH