Not sure I believe the dirt cheap idea. Plenty of medications that seemingly should be cheap aren't. And yes I get that qc/qa is really important for things we consume but that's not the entire reason for the price discrepancies.
I'd think for some drugs it would tend towards the painkiller/heroin structure that exists where it was easy to get a prescription or buy from someone who had one. Assuming you have enough money, but if not you go into the second tier. Also it might be harder to get painkillers than it was, I've stayed away from friends in that scene for a while so my exposure's pretty limited now.