With any black market, it's impossible to track the full extent of its reach. One way to estimate the relative quantity of various drugs in the city is to track undercover drug buys and seizures by the police department. This system of measurement, however, is an imperfect science; large busts (see: cocaine, 2009) skew numbers for a particular year, throwing off the curve, and budgetary and tactical considerations can restrict potential drug buys.
Note: Some of the drugs listed below are generally not measured in grams - marijuana, for example, is measured in ounces - but all amounts have been mathematically translated into grams for comparison's sake.