Everything depends on the feature-set.
It has to be better than Torque or have a better feature set when it costs more.
The problem is, an iPhone without an OBD connection kit won't give good results.
But with it, it becomes more expensive ... and a lot less people will buy it.
The free Garmin stuff in their GPSes is rubbish for FE.
It derates the driver for driving below the limit, and for accelerating briskly near BSFC.
If it's that expensive to simply get it started, I wouldn't bother.
@ 5 Euro / 6.25 USD per download it'd take well over 3000 downloads just to break even.
This ain't no angry birds
Also, it's not guaranteed that people will get the claimed reduction in FC.