Giving the game away for free, Burnell said, can actually make more money than charging for it outright. “The vast majority of people pay nothing at all,” he admits. “But those three or four people are paying maybe $100-plus sometimes.” When the number of players gets up into the millions, that can mean big money. When it’s all added up, “you have a revenue model that's actually making more money that charging for the app up front,” Burnell said.

UAA psychologist Dr. John Petraitis said the model isn’t necessarily new. It’s a well known “foot in the door” marketing trick familiar to car salesmen. Getting a customer to agree to small requests can eventually push them to say yes for a big sale.

“So [players] spend that 99 cents, and then they spend more time playing the game, and are more likely to spend another 99 cents down the road, and so you get this escalating commitment of behaviors.” And once those purchases are made, Patraitis said, players are more likely to justify their investment by playing the game even more.

And when a purchase is just a tap of a touch screen away, spending a little extra to win is easier than ever before.