A lot of those cheap mounts are made of cheap cast aluminum or cast zinc, depending on what grade of aluminum it can be okay or cheap junk. For example the Trijicon Reflex is made of Investment cast Aluminum, which is designed properly to be strong in a cast application. On the same token, bar stock 6061 Aluminum without the heat treatment won't last very long either (6061 and 7075 aluminum alloys get their strength from the T6 heat treatment process)
The good quality ones are full machined steel or aircraft grade aluminum, and the attachment points/claws are much better quality. You may be able to use the cheap-o mount on the HK, but it very well could crack or break or bend. Good companies like B&T or MFI make good quality stuff, so stick with them IMHO.
The cost difference is with what they use a lot of times, like I said before the cast zinc is pot metal, good for making pins for shirts and stuff... cast aluminum can be more difficult to get a strong casting alloy, as it usually requires a lot of silicon to get it to flow properly (which weakens the alloy). However for a .22 or for airsoft stuff, it's usually adequate.
Bar stock machined steel and alluminum take a LOT more time on a CNC machine but you get a vastly stronger product, and one that can take repeated shocks without weakening.
