You mean, "I sell an app for money, then I spend less than I make." This seems like a quite different business model than what's he's trying here, with a free app but paid subscriptions and paying authors and such.
The Instapaper model is much simpler... but it's clear Marco doesn't really like it much any more, since you only make money by selling the app once but it costs you to support those users forever. So "spend less than I make" eventually gets very difficult if sales trail off.
This is totally the same as Instapaper but instead of an app, it's magazine issues. All paid-subscriptions magazines still appear as free apps on iOS, but they either offer a free sample or a free trial.
In this case, it's a free trial, which makes sense when it's the first issue! But don't be surprised if The Magazine switches to a free sample. That's especially worth noting if you're expecting to optimize for a free trial.
My point is that Marco doesn't have any expertise selling advertising, as many would like him to do, and does have a lot of experience selling through the App Store. He's just keeping the business model as constant as possible.
The Instapaper model is much simpler... but it's clear Marco doesn't really like it much any more, since you only make money by selling the app once but it costs you to support those users forever. So "spend less than I make" eventually gets very difficult if sales trail off.