Original price unknown
The SC400 "4 & 4" cassette deck is part of a larger set of instruments made by Yamaha for the schools, called "Yamaha music laboratory system". Yet details are extremely scarce: a manual for the MLT201 cites the possibility of integration with the SC400 deck, so they were probably part of the same set built in 1981. It's unlikely that it was ever sold to the public.
It basically works like a 4-track recorder and player, very similar to the MT1X: the cassette can hold up to 4 mono tracks, and Yamaha probably already supplied lessons on music cassettes. Track 1 contained the main melody, track 2 the basic harmony, track 3 a basic rhythm and track 4 the piano accompaniment. Using the SC400 the teacher was able to individually control each of the 4 channels when playing the cassette to the students. Individual channels can be routed to the left or right channels as well.
The system can hold up to 16 students (each one having their own piano linked to the teacher's control board / piano) and this seems to be reflected with 16 buttons on the front panel but it's unclear what was their purpose since there aren't individual sound inputs or any kind of data connection.
The design language is very reminiscent of some of the early Portasound keyboards like the PS30.