Mexico City (November 2020):
LSD: 7-10 USD for a tab depending on potency, a sheet of 25 tabs can go from 75-200 USD depending on potency.
Mushrooms: 50-100 USD for an ounce of cubensis depending on the seller and season, a single gram can go for 5 USD.
DMT: 40 USD for a gram
Ayahuasca: the plants for preparing ayahuasca can be obtained legally in Mexico, from 40 to 75 USD depending on the seller.
Salvia Divinorum: 10 USD for a big chunk on salvia leaves, extracts can go up to 50 USD for a couple grams for the most potent extracts, legal in Mexico.
MDMA: 5 USD an average pill (often cut with meth or other studf) 7-10 USD for a "premium" pill (pills bought on the deep web often pure), buying in bulk premium pills can be bought from 4 USD each, a gram of pure MDMA can go for 40-50 USD.
Heroin: 100 USD a gram although almost impossible to come by in Mexico City.
Cocaine: 25 USD for an average gram, 50 USD for premium grams.
Marihuana: 5 USD for brick weed, 15 USD for a decent quality ounce, up to 100USD and higher for premium/imported strains.
Oxycodone: Pharmaceutical boxes can be found on the black market at pharmacy prices although they are almost impossible to find, 100 USD for a box of 30, 20mg pills.
Morphine: Again almost impossible to come by, about 11 USD for a box of 30mg morphine pills with 20 pills.
Benzodiazepeines: a box of clonazepam can go as low as 4-5 USD for a box of 30 pills of 2mg each, sold individually Alprazolam and Clonazepam are sold for about 1 USD a pill.
Opium: Extremely rare to find so the price is a quote from 1 individual seller, 100 USD for half an ounce of opium, prices vary widely, with a KG of opium costing as low as 300 USD in rural areas where it's harvested, considerably more expensive in cities.
There are other ones which I have not mentioned due to a lack of information regarding the price as I do not consume them, such as Meth, Crack, pharmaceutical stimulants, etc... hope this helps someone out or at least is entertaining to read.