US iPhone users spent, on average, $79 on apps last year, up 36% from 2017

Per TechCrunch:

As is typical, much of that increase can be attributed to mobile gaming, which accounted for more than half of this per-device average. However, more substantial growth took place in the categories outside of gaming — including those categories where subscription-based apps tend to rule the top charts, the firm found.

I bet app spending is going to go up each year for the next few years, partly because people will spend less on hardware upgrades, and partly because software is becoming way more important than hardware in mobile now that the market has matured.