For DC performance testing Atlassian recommends some tools such as JPT and E3. You might find yourself unhappy with these tools and feel the urge to build something yourself like others have done or use another existing service like locust or gatling. However the tools that Atlassian recommends handle the whole AWS templating stuff for you (i.e. they get a Jira/Conf DC up and running for you) so that is something you need to handle yourself if you choose to use different tools (which you might want to do, depending on how and what you want to test).
The whole process can be very time consuming especially if you have no experience with it. As for AWS costs there’s a good chance that you might rack up a few hundred dollars until you get everything up and running the way you want - but the more expensive factor will be the time you spend on it.
Since your app is free I’d probably reconsider if the whole DC verification process is worth it. 