Well if only 5% of the people who bought the game (2.38 million) end up buying DLC, that'd be 119,000 units. At $10 a pop, that's 1.19 million dollars.
And that ought to cover it easily, I'd say.
The engine is built, the marketing is done, there's no manufacturing costs - I'd say DLC is relatively cheap compared to games. For something the size/scope of Knothole Island? 100k? 150k?
Remember that studies show DLC helps push more game sales as well to a slight degree, so that's profit too.







