How to get money from your audience remains the core issue for the games industry.

At the Independent Games Summit at GDC 2013 in San Francisco, Shane Neville from Ninja Robot Dinosaur Entertainment spoke on the subject in his talk Designing In-App Purchases Without Losing Your Soul.

Neville was part of the indie team that developed the successful iOS and Android game Shellrazer.

"You are a giant turtle, with three guns on your back to blow stuff up: it's a multi-touch shooter with RPG elements," Neville explained.

Revenue models

Interestingly the game was originally released as a 99c title with a virtual currency and IAP, before going free with a $2.99 unlock barrier (and with IAP).

Neville said that 8 percent of players used IAP, with 30 percent of overall revenue coming from IAP.

"That revenue helped me to become a full-time indie developer," he added, stressing the important of the revenue source.

As for his design approach, Neville said a key view was to ensure the game was designed so that players with skill, players with time, and players with money could all get a good (if different) experience.

A big element of this was that the game had a lot of complexity and depth, particularly in terms of mixing up the weapons management and multitouch dexterity as you're in control of three different weapons systems at any point.

The balance

When it comes to players with plenty of time, Neville said you can play - or grind - through the entire game in one ten hour sitting, if you want to.

'A lot of free-to-play games have a time gate that forces players to stop. We don't do that," Neville said. "We're really generous with our rewards for people who have time."

More generally, purchases in the game are optional.

"We only prompt the players to go to the store unless they have run out of money [virtual currency]," Neville said. "We got 30 percent more revenue from IAP, but not one English language review complained about IAP."

Heart of the matter

Taking a more ethical angle, he said Shellrazer's developers didn't use the term 'whale', and that - in general - developers shouldn't design games for players who they think are fools in terms of the way they manipulate IAPs.

"We built our game for players with skill and players with time, but enable players with money to spend, " Neville said, revealing that the maximum amount of money that any player should spend to get everything they required for gameplay reasons in Shellrazer was $50.

"I believe if you design for IAP alone, it's a slippery slope," he ended.

"Business models don't determine ethics, the developers do. And dirty tricks work. That's why companies use them."