It is my guess that the CheaperRFID is literally a guy in his basement, maybe even a college kid, who is doing it for fun. The electronics works but the mechanical is lacking. For example the 40m board literally sits in the enclosure and is NOT fastened! There is also no documentation, not even a one page readme that gives things like voltages, or a warning that the parasitic power for the reciever won't work on a USB RS232 thingie. The PCB's don't have things labeled, etc. I will be pleasantly surprised if they work over temperature and vibration.
I'd bet that the $300 units are engineered, documented and tested in much more detail not to mention the fact that they do distance/signal strength (which presumably means they have an AD to measure the signal).
I'm not saying that I think the more expensive units are worth the cost, just that it is not an apples-apples comparison.