89 Euros= £64.
That is rather a lot of money...even for Dr. Schaarwachter's expertise :(
Sadly it's probably a reflection of the relatively restricted readership for such material - weighed against the very high production costs of a musicological analysis which includes extensive musical examples.
I would imagine they are price-pointing these volumes with the institutional, university and library market in mind - rather than the small number of private individuals who would spring for such a purchase.