According to this article, cassette sales went up from 74,000 in 2015 to 129,000 in 2016 thanks to specialty cassette albums from Justin Bieber, The Weeknd and a few others. Hopefully more artist will continue this trend.
according to the engadget news: report only reflects US market: Vinyl albums sold 13.1 millions in 2016 Cassette tapes sold 129000 copies in 2016 (74% increase from 2015) The Guardians of the Galaxy: Awesome Mix Vol. 1 sold about 4,000 copies each in 2016 and 2015 Justin Bieber album sold 1000 copies