They already did the overwhelming majority of the work. Developing that kind of stuff is expensive. I've seen estimates of ~100MM USD quoted for the discovery of a new drug.

As others put it, the manufacture of simple chemicals in the quantities involved in pharmaceuticals is effectively free. As in, give me 100k and I'll be able to make enough of the drug for the whole world for the next hundred years. The packaging of most pharmaceuticals is more expensive than the manufacture of their contents.

The cost is overwhelmingly in R&D -- finding the chemicals, proving it doesn't kill people, and proving it actually does something useful.

Don't forget a huge expense: Battling lawsuits (mostly frivolous) from users of the drug and potentially paying out huge settlements.