Check how the app is funded

There are perfectly trustworthy free and open source programs out there, but not all that many—most apps are either funded by an up-front payment, an ongoing subscription, or adverts that may or may not be targeted around your location, interests, and anything else that the app has managed to get out of you.
It can be the case that apps that are free or very cheap are taking more from you in terms of data collection, and putting more ads in front of you. Be cautious with free apps that don’t have any obvious source of funding, especially if you’re seeing other red flags as well—you might be paying in ways that you don’t realize.
At the same time, you don’t want to be paying over-the-odds for something that’s not worth it or even malicious. There’s no right or wrong approach, but what you’re paying for an app and how you’re paying for it are other considerations to weigh up alongside everything else we’ve mentioned.