But marginal cost for whom? The entity manufacturing the drug, the entity that holds the inventory, the entity that owns the IP, or the entity that the pharmaceutical rep works for?
Any sufficiently large corporation is more likely than not to be an amalgamation of interconnected subsidiaries, for a wide variety of purposes. Once you're at that stage, there's a lot of room to get creative with how you structure intra-company charges to form whatever optics you want. It's entirely plausible that one internal entity may charge the marketing entity full retail price for samples, if they feel it's beneficial and have enough justification to satisfy accounting and auditors that it's a fair market value for the product (which list price would generally satisfy that requirement).
Any sufficiently large corporation is more likely than not to be an amalgamation of interconnected subsidiaries, for a wide variety of purposes. Once you're at that stage, there's a lot of room to get creative with how you structure intra-company charges to form whatever optics you want. It's entirely plausible that one internal entity may charge the marketing entity full retail price for samples, if they feel it's beneficial and have enough justification to satisfy accounting and auditors that it's a fair market value for the product (which list price would generally satisfy that requirement).