Often drugs and other chemicals can be produced very cheaply in large quantities once:

1. The compound was synthesized in the laboratory
2. The compound was tested. This is a major effort that takes a decade.
3. The compound was approved. This is another major effort.
4. A large scale synthesis process was developed. A problem for chemical engineers.
5. The production line for this synthesis was actually built and tested.
6. The drug is produced in large quantities at low unit price.
7. The drug produced by this line is tested and approved for sale.

If you only want to consider step #6 above, the unit cost of production is often really low, being only the cost of the precursors, electricity, production staff, etc. This is the MARGINAL UNIT COST. But, if you consider the TOTAL cost, it is vastly greater.