Ramin Shokrizade writes on Gamasutra about the various tricks used by developers of free-to-play games to get your money:
Skill Games vs. Money Games
A game of skill is one where your ability to make sound decisions primarily determines your success. A money game is one where your ability to spend money is the primary determinant of your success. Consumers far prefer skill games to money games, for obvious reasons. A key skill in deploying a coercive monetization model is to disguise your money game as a skill game.
King.com's Candy Crush Saga is designed masterfully in this regard. Early game play maps can be completed by almost anyone without spending money, and they slowly increase in difficulty. This presents a challenge to the skills of the player, making them feel good when they advance due to their abilities. Once the consumer has been marked as a spender (more on this later) the game difficulty ramps up massively, shifting the game from a skill game to a money game as progression becomes more dependent on the use of premium boosts than on player skills.
Note that the difficulty ramps up automatically for all players in CCS when they pass the gates I discuss later in this paper, the game is not designed to dynamically adjust to payers.
If the shift from skill game to money game is done in a subtle enough manner, the brain of the consumer has a hard time realizing that the rules of the game have changed. If done artfully, the consumer will increasingly spend under the assumption that they are still playing a skill game and “just need a bit of help”. This ends up also being a form of discriminatory pricing as the costs just keep going up until the consumer realizes they are playing a money game.
As with gamification, the world of free-to-play games is a fantastic example of the weaponisation of behavioural economics. There's a whole group of people for whom books like Daniel Kahneman's Thinking, Fast and Slow are something between an instruction manual and their own version of The Prince – and they're making video games.
The tactics listed are why I've found myself increasingly suspicious of F2P games where the purchases have any effect on gameplay at all. For instance, I loved Kingdom Rush, a fun little tower defence game for my iPad. But the sequel introduced payments for one-off bonuses as well as new heroes which you could buy. Now, whenever I fail a level, there's the sneaking suspicion that the level was designed to be impossible to finish without splashing out. Who would have thought that we'd be nostalgic for the days of paying for a game and then playing it?