Corpus based teaching is a dominant practice these days. We must, however, be cautious of its consequences of becoming too narrow or too broad. Detailed investigations are needed before we validate it. Requesting comments.
Unfortunately, COCA is not freely available -- it's quite expensive. You can make a limited number of database queries on the COCA website for free, however.
It's free and very easy to use at an introductory level. It may be expensive to purchase elements that are for sale, but the corpus itself is free. And it's very easy to use in the classroom.