Totally depends on the magazine. Typically extremely expensive. Unless you're a big dev it's not worth it.
For a small time app I can almost guarantee there would be a good roi.
Agree - they're going to be expensive. Things to consider:
Does your app match the target audience of the magazine? For example, you have a "to-do" app and the magazine is targeted to mothers.
Does your app have current positive reviews? In general, you don't want to advertise your app if it's currently a 2 or 3 out of 5 star app -- it will cause people not to purchase and most likely lead to a bad ROI. Wait and improve your app until it's at least a 4 star.
Do you have good supporting marketing? I.e., a good website, good social media, etc;? A print ad is one part of the equation. Make sure someone can quickly google your app, find a high-quality website with great screenshots and videos, and quickly link to buying the app now.
Best of luck.