Well, need to take into account that most of the more expensive ones are the 'named' or pre-customized variants of basic weapons. ie: That Gun for a .556 pistol, or Chopper (?) for a cleaver (not that you can mod a cleaver that I know of). Since they come pre-customized/modded, it makes some sense that you couldn't do it further. think of it this way. Because it's custom, not all the standard/default bolts/screws/etc are in the same places anymore, so the 'standard' mod just can't be attached properly to be useful.
Sorry, by expensive I just meant the weapons like the Marksman Carbine or the Anti-Materiel Rifle. I agree that you shouldn't be able to modify "named" weapons. They're already powerful enough.
Any by the way, you can mod a cleaver. Replace the handle with a carbon fiber body (to decrease weight), add a nifty laser (to aid in aiming it), and maybe a scope (so you can see your enemy that's right in front of you). Would be kinda pointless to mod it in real life though.
