The folks at 140 Proof write:

People increasingly prefer free, ad-supported apps for their tablets and smartphones, yet many developers still aren’t sure how to tackle the free vs. paid issue.

Deciding when to charge for your app, and when to try an ad-supported model, is one of the hardest decisions developers must make.

It turns out most users are reluctant to pay for mobile apps, so the best practice is to develop both a free ad-supported version and a for-pay no ads version.

By the way, there are ads that protect users' privacy by using the Interest Graph instead of vertical targeting and cookie tracking.

Conventional wisdom says it's easier for developers to make money from mobile apps than from web apps.

We'll see if that actually remains the case over the next decade.