In these notes, the authors state that:
On the Concorde, a one count drag increase (∆CD = .0001) requires two passengers, out of the 90 ∼ 100 passenger capacity, be taken off the North Atlantic run.
and refer to this article (paywalled). I've seen other statements that an increase of one drag count is equivalent to 5-10 passengers, but do not recall the source.
Do you know of any other references that list the equivalence between drag count increases with reduced numbers of passengers?