I have lots of music which I got basically for free, and a lots of music for which I paid.

My general attitude towards music is that I’m in it to explore, learn and enjoy the indescribable human connection that I get through, and only through listening to someone’s little “message in a bottle”. Doing this for many years (I’m 44 so over 30 years now) I’ve learned that training my brain on multiple styles and genres, the connection can always get deeper and more rich.

So I would not be able to ever say a purchase was not worth it, because I would always assume such decision to be rushed. I had too many experiences when I listened to something, didn’t quite like it, but later I somehow “grew into” it, and then I learned to love it for years and years.

Different music ends up playing different roles in my life. Some albums end up teaching me a genre or a style, some end up acting like a gateway drug, some end up as a “stand in” for whole genre. Some end up as “holy relics” of who I was, and are re-visited from time to time to see whether I’ve changed and how. Some end up on a shelf and get re-discovered, some end up on a shelf forever.

(That “stand in” part is kinda tongue-in-cheek, but it sometimes almost works; eg. I would never set out to get a Dub album, but Dub Guerilla is one of the 100 best things I’ve ever heard, and it’s just so darn satisfying that it satisfies all and any of my Dub needs.)

Sometimes my brain can be just really petty about things, like completely disregarding an album because of a track or a section which I feel is a mistake. Sometimes I just know I will need much more time, sometimes I feel certain things might remain hard to get into maybe forever.

Don’t get me wrong, somewhere among those piles, there are really things that I won’t ever care to pick up, and perhaps would not purchase them again, but it almost never has much to do with number of listens. It might be things that I just got with bad expectation (ie. not listening upfront) or things that I enjoyed because of content (eg. lyrics) but I have changed and moved on.

Other times music is best experienced live, for some bands the “spirit” simply cannot be tamed, let alone reproduced. Sometimes I get album from band directly after a show and then end up never feeling it again from the CD, but then again, I would say it was not worth it, because it’s still a great way to “tip” the artist, and sometimes it will just work out.

(A bit more related to the OP: Incidentally, just today I broke my all-time record by spending about 20 EUR for V​í​n by Janus Rasmussen, and for reasons completely unrelated to OP and the price (but related to a sub-thread) I also did something I would never do normally–put one of the tracks on repeat for several hours. That’s not to brag about money–it’s just funny how it ended up accidentally as “$ per listen” experiment, although )

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If I get to keep the music in mp3, flac, or some other file format? Even a single play is worth it

You don’t “buy” music. You buy a license to listen to that music for as long as the license issuer still owns that particular song.

So never. Pirating music is the only path to ownership.