Comment by tsujamin
The standard I arrived at is roughly "would I be sad if, in 15 years, I forgot about this book/piece of music?". If it's something that I enjoyed so much today that I'd be afraid to lose it amongst 10,000's of eBooks or songs on a streaming platform, I physically buy it.
I like this. I do something similar; I ask myself “Am I buying this for the knowledge inside it, or the experience of it?”
I buy the latter. Be it a comic book, photography book, or something else. Those are artistic experiences for me.