I was under the impression that the biggest driver of marketing costs for drugs was providing free samples of the drug to doctors, with training doctors coming second and advertisements a distant third.

Depends on the drug and the market. Not all drug have a free sample program, but yes, it can add up since they account for the full price, not cost of goods.

Personnel can be a big chunk as well, since the fully loaded cost of a sales rep can be $400k per year and for primary care drugs, it’s not unusual to have 1,000 reps.