I hear again and again questions on how to get a good, cheap microphone for doing video recording. People will usually forgive poor video quality but they have no patience for poor audio quality – especially for educational content.
I can show you a great mic for about $20 US and it will work with a phone, camera or computer. If you need a USB microphone, you can add a USB input to it and still keep the price under $30 US. But you may already have a suitable microphone and not even know it!