Some more useful numbers from Buying D&D:
Charles Ryan last year, when he still worked for WOTC posted that their average hardback sold in the hundred of thousands. Be advised that retailers and distributors claim about 55%-60% of the retail price on the book. The printer and publisher get the reminder. IME WOTC collects around $9 on each $30 book, x 100,000 = $900,000. Not bad even if their developments costs are over $250,000 (my estimate) on each book (3-month development cycle I believe they use) and not including other overhead costs.
You can see how RPGs might scale (or not), depending on development time/effort, size of print run, and sales vs. expected sales. A book's development is a fixed cost -- although a book with more/better development (writing, art, etc.) should sell more copies -- and its print run has to be more-or-less decided up-front, so, with all that operational leverage, profits can end up all over the map. Selling 10% fewer copies than you hoped might mean no profits, and selling 10% more might mean doubling your profits.