Economically speaking, any product must sell enough to cover at least the explicit (easily traceable) costs of producing it to be considered profitable (and thus marignally successful). As such, you should gauge how successful a game is not by a set value, but by a simple formula:
Profit = revenue from sales - explicit costs
Let's take a hypothetical situation of a game that costs $12,000,000 in explicit costs to produce, advertise, and publish (as well as all other explicit costs). If $40 profit is made per sale (split between publisher and development studio), we can divide 12,000,000 by that to get the break-even point (300,000 sales). Thus we can say that a title which costs $12,000,000 in explicit costs that gains $40 in profit per sale must exceed 300,000 sales to have been marginally successful.
For a title to be truly successful, implicit costs (what you sacrifice to make the product) must also be exceeded. Let's say the $12,000,000 title above has an overall implicit cost of $4,000,000, added onto the explicit costs to give a total of $16,000,000 to release the game. In that case, 400,000 units would have to be sold to cover both types of costs, and over 400,000 units sold would be true success.
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