Angry Birds is neither innovative in its design nor groundbreaking in any technical sense. Its biggest contribution (apart from sheer fun) is the absolute certainty that mobile games at alluring prices can sell millions of copies and generate astonishing revenue.

Angry Birds is the number one iPhone and iPod touch game in Apple’s App Store in the “popular” and “highest grossing” categories. It’s slowly introducing itself to Android users, too. The game has sold at least 6.5 million copies to date. It’s usually priced at $0.99, but even at that price we’re talking about substantial revenue.

The game has sold well because it strikes that perfectly delicate balance between “simple enough for anyone to pick it up and play” and “challenging enough to keep players hooked for hours.” Most of the great, best-selling games on any platform — mobile or otherwise — succeeded because they hit that sweet spot. But there’s also a balance between value gained and money spent. At $1, most people are willing to give the game a shot.

That helps it sell more copies than a pricier title would. The game’s Finnish developers could afford to sell it for such a low price because casual gamers don’t expect amazing 3D graphics, which are expensive to produce. They just expect fun, and if a developer has a great idea, that can be comparatively cheap to turn into a reality.