This is a bit like comparing street amphetamine powder with prescription adderall. People are willing to pay for a pure product measured precisely in laboratory, not cut with other dangerous chemicals delivered as a pill.
In the specific case of ketamine, it’s easier and cheaper to acquire, and generally goes through far fewer intermediaries between the pharmacy and the street consumer. So the stuff you find on the street tends to be dramatically more pure than, say, cocaine.
From what I remember from the Hamilton's Pharmacopeia episode on Ketamine, this synthesis is actually quite complex and a high-yield synthesis (which is presumably being used currently by manufacturers) is not published in publicly available literature.
However, ketamine is widely used as an anesthetic, so I think the "cheap to make" refers to Ketamine redirected from the pharma market to illicit channels, in which case "making" ketamine is just drying the solution to crystalize the sole ingredient.