Well, if you're just interested in breaking even the margins should chew up the cost of printing extra boxes. Say that the profit is $15/unit (of course, I'm assuming that your labour is worthless ) Then 20 units sold ought to pay off your expenses.

Though, if I may suggest, I saw a few places selling jewlery boxes with your imprint on it. It may make more sense to get a "classy" box with a silver company imprint (rather than the product name), then include a manual that is printed with a glossy colour cover (including art based upon the game's theme) with laser printed pages inside. The covers and manual pages would be printed in smaller quantities, as needed.