I was able to use department letterhead for some of my mag needs, but as a collector I had to scrounge the market for everything else.
Pre-ban mags could range from 60 dollars (used 226 9mm) to $160 (rare G20 pre-bans).
Retailers like CDNN still had pre-ban mags for various pistol models on sale up until near the end of the ban, but they usually ran from $99 dollars and up.
Some were just hard to get no matter what. You just had to watch the auction sites and hit the gun shows.
As far as rifles, well that varied widely. Typical prices for most pre-ban rifles were double what they had been previous to the ban, but it could vary. Most any pre-ban rifle could still be found if one looked hard enough, but cost was a factor.
Instead of building newer and larger weapons of mass destruction, I think mankind should try to get more use out of the ones we have.