If you have access to the D20 Weekly online magazine put up by Steve Jackson Games, there is another excellent cost breakdown (with an Excel spreadsheet attachment available for use) article from John Nephew on the cost of producing a D20 supplement. It includes everything from writer and artist payments to expected profit per book.

Not 100% useful for folks developing their own games singlehandedly but a good breakdown for the rest. Especially with recent talks of D20 profitability.