Just swap with other developers" is the most common advice for Google Play's closed testing rule, and it is good advice. It is also usually given without the price tag.
Free testers are not free. You pay with your own time, every day, for at least 14 days. Here is what that actually costs, using the real numbers from PeerPlay's free exchange, what you get in return beyond the tester count, and the cases where paying is the smarter choice.
The daily math
For newer personal developer accounts, Google requires at least 12 testers opted in to your closed test for 14 days in a row before you can apply for production. In a 1-for-1 exchange, one tester for you means one app you test for someone else. So 12 testers means 12 apps on your phone.
In PeerPlay's free route, each tester opens each app from PeerPlay for about 30 seconds a day. Twelve apps at 30 seconds is six minutes of app time. Add launching, switching and the occasional slow app and it lands somewhere around ten minutes a day, as a rough estimate rather than a measured number.
Over 14 days that is a couple of hours in total. And because you should recruit more than 12 to protect your count, you will probably test a few more than 12 apps, so budget a little extra.
The costs nobody mentions
Storage. Twelve or more extra apps take space. Most are small. Some are games, and one developer recently asked me about a 1.6 GB game, which is the kind of download some exchange partners simply skip.
Consistency. The time is small, but it is every day. Miss a few days and you can drop out of someone else's test, which is exactly the thing you are afraid of happening to your own app. A daily reminder at the same time solves most of it.
Timing. Your partners' 14 days and yours rarely line up perfectly. If you joined someone's test a week after yours started, you keep testing their app after your own test is done. That is the deal, and good exchanges depend on people honoring it.
What you get besides 12 testers
The tester count is the obvious return. The less obvious one is that you see a dozen other indie apps up close, on a real phone, for two weeks. You notice which onboarding makes you want to keep going, which permission prompt makes you hesitate, which screen is confusing. That is a free lesson in what your own users will feel.
Your testers are also developers. They know what a useful bug report looks like, and some will send you one. And if you leave real feedback on their apps, they are more likely to return the favor with real feedback on yours, which is what Google's production application asks about.
Finally, you meet people with the same problem. The developers you swap with now are the ones you can ask for a favor at your next launch.
When it is worth it, and when it isn't
It is worth it when money is tight, your app is a normal size, and you can spare ten minutes a day for two weeks. For most solo developers on their first app, that is the right trade.
It is not worth it when you have a fixed launch date that someone else depends on, when you cannot commit to daily testing for the whole period, or when your app is so large that exchange partners will drop at the download. In those cases a paid campaign, where testers are paid and you do not test back, buys you certainty with money instead of time.
Either way, the rule underneath does not change: 12 testers who really stay for 14 days. The exchange just decides whether you pay for them in minutes or in dollars.