How much does a game have to sell to warrant a sequel?

[Dolla Dolla]

Depends on the budget. It's different for every game.

Assuming it gets back the costs it took to make the game, and makes the company a nice profit, the chances of them revisiting the game is much higher than if it failed.

For instance, selling 100k Zack and Wiki is probably a success, whereas selling 100k of a high profile title, like say Bioshock, would be an unmitigated failure.