I've also done a ton of cost estimation and I thought this was a reasonably good approach for capturing the "core cost" of development of an app.
What it doesn't capture is testing beyond just design-code-unit-test such as multi-device testing, user testing, user acceptance testing, etc. and all of the change management that goes hand-in-hand with that.
What it doesn't capture is testing beyond just design-code-unit-test such as multi-device testing, user testing, user acceptance testing, etc. and all of the change management that goes hand-in-hand with that.