Volumetric measurements aren't really good for drugs, the density of the drug may vary depending on quality, origin or manufacturer, in the case of crack and meth. Weight is always better, but then you measure with half, quarter, eighth and sixteenths of ounces. See, we do have to use fractions after all.
Fun fact, in America, a ton is 2000 pounds, which is slightly less than a metric ton. In America if you order a ton of bricks, you'd get less bricks than you would if you ordered it in France.