They were an enormous thing at the time, as you say especially money-wise: specially fitted classrooms, furniture, maintenance contracts and the structural- situational course tapes all played a part. Probably worth it though, most people I knew at school loved going in there recording conversations etc.
I always loved the aesthetics of those rows of booths, the mechanical sounds, the whole 'procedure' involved in their use.
The Audio Active Comparative system used in langauage lab booth decks is basically overdubbing under another name! I too recorded lots of my music stuff on modified language lab cast offs used like a 'portastudio.'
Since both my parents worked in education, in the 80s I had a seemingly endless supply of bits as language labs were replaced by computer suites filled with then-popular BBC micros.