Attached to a wall at Ustwo Games' London studio is one of the team's favourite user reviews for Monument Valley.
It says: "Thank you for making this game, I thought it was really beautiful, one of the best things I've played on mobile, and it really impacted me. I just thought 3.99 was too much."
"That's always been super funny to us," chief creative officer Dan Gray tells
Amusing as it is, it illustrates the biggest challenge of the mobile space: the value players place on games. Following the launch of the App Store, a race to the bottom means that the vast majority of mobile titles are now free-to-play..
Premium is still an option, of course, as Ustwo has demonstrated with Monument Valley -- the sequel to which made $10 million in its first year. But Gray acknowledges this is not the norm.
"Making premium games has been a dangerous proposition for a number of years," he says. "Even we know we were very lucky with Monument Valley. We were lucky to get a unicorn, but the exception doesn't prove the rule. It would still be dangerous for us now to throw a game out there and charge 5."