When Dean Moriarty
, he probably paid $10.00 for a "can" - a one pound coffee can stuffed with stuff.
In the sixties, pot began to be sold by weight, not by volume. In those days, the $10.00 quantity was one ounce. In the parlance, this was usually a lid, but it was sometimes called a "can", in deference to the Beatnik jargon.
The $10.00 per ounce price point persisted for a good many years, and with it, the slang "can" for an ounce.
As inflation eroded the notion of $10/oz. pot, the slang "can" drifted away with it. Now it's as relevant as "tea" or "boo" for marijuana. Times change.