An Ed visa doesn’t allow you to teach. As the name sort of implies it allows you to study. Which is something you should do before suggesting that people can do illegal activities.
I’ve seen posts saying 400 USD to apply. That’s about 13.5k. There’s probably some place on earth that charges 15, but as I don’t know what each individual embassy will charge it was an approximate figure.
The visa when granted is valid for 5 years. At this stage it appears that no one is checking whether you actually do what you signed up for, but perhaps in the future they may. There is nothing that states you must continually do courses for the entire 5 years.