What is the best way to cost effectively get rid of bugs before launching an app?

NASA is famous for writing the most bug-free code in the history of programming. They do it by being fanatics about test-driven development. They write tests for tests. If you want to have a rock solid app, you have to put everything else aside and focus only on quality. Every time a new build goes out, every single member of the team has to test every tiny nook and cranny and write tests to mimic how they're testing it in real life. The goal is to start small with a solid core and get it functioning error-free before building other features. If you hold yourselves accountable to make feature incredible before you begin building others then you'll never have to go back and clean up something later. In short, try to do it "write" the first time :)

Try to cover basic functionality with acceptance tests at first. There are a lot of tools that doing that. Better to use tools that requires programming skills to write the tests. But if thinking about test automation in long-term is better to start writing test even before implementation. But most of the time that requires mindset switch for all the developers. Test-first, not test-after.

I always find that the common user is the best approach to testing and finding bugs. I have developed many web and mobile apps and for the last 8, I gave the app away to 40 people. If you give the app for free, people will be glad to test it and they will find more bugs than any technical QA testing person since QA techs think technically and not the way the common end user would.

I have found that the common end user is most rock solid approach and it is free.

Bruce