Ask HN: Measuring CPA with mobile apps

Hello HN,

I'm really curious to understand the different tips and techniques you use to measure CPA (Cost Per Customer Acquisition) for your startup business.

My business is focused on delivering a set of mobile products to a distinct (almost niche) user base and I'm on a mission to really understand my key metrics, CPA being the first. However as we deliver our solution via a third party (in this case the app store) I feel that we are are at a disadvantage to our SaaS counterparts.

If I look at our customer flow, its a pretty familiar story starting with some lead generation activity (in this case Google ads):

Adwords > Dedicated landing page > Link to App stores > Download / install > Register

Now I can measure everything in detail up to the point the user leaves our ecosystem and ends up at the Apple or Google store. So I am curious, how do you manage this if at all or am I expecting too much?