Count me as another big fan of ECM. But here in LA, at all the brick and mortar stores I shop at, they tend not to be in bargain bins. They tend to go for about $9.
Record stores are well aware of their sonic and musical attributes. So, even though they have a limited audience, stores know fans will pay good money for them.
The good thing about used ECM, is that since they always appealed to audio and music enthusiasts, they tend to be well cared for.
Another series I like a lot, musically and sonically, is the "Spectrum New Music" series on Nonesuch records from the 60's and 70's. While Nonesuch has a undeservedly bad reputation for noisy vinyl (I don't find that to be true), the imaging and soundstage on these is frighteningly good.
I can close my eyes, and very vividly 'see' the musicians in a reproduced acoustic space. I feel like I can get up and walk among them.
Now, musically speaking, these are quite 'thorny ' sounding, avant-garde classical music (typical of post 1950 classical music), so, YMMV.
Composers such as: Elliott Carter, Joseph Schwantner, Druckman, Wolpe, Wuorinen, etc. Some pretty 'difficult' stuff.