Games and the formula that leads to their success can be a bit of a mystery, perhaps more so on mobile. One of 2014's big hits, both in terms of critical and audience success, was Monument Valley by ustwogames. The isometric 3D puzzler was breathtaking in terms of its visual design and had nice mechanics, and it managed to earn nearly $6 million in revenue from over 2.4 million individual app sales. But the cost of development might surprise you, as well as the completion rate for the game's…