Percentage costs until patent expires.

(3%) 1. The compound was synthesized in the laboratory (along with many other useless compounds)
(50%) 2. The compound was tested. This is a major effort that takes a decade.
(25%) 3. The compound was approved. This is another major effort.
(5%) 4. A large scale synthesis process was developed. A problem for chemical engineers.
(10%) 5. The production line for this synthesis was actually built and tested.
(5%) 6. The drug is produced in large quantities at low unit price.
(2%) 7. The drug produced by this line is tested and approved for sale.

In other words, by my guess, only 5% of the cost is running the production scale synthesis operation.