I am in no way saying that this is cheap but 300 TB will set you back a little less than $6k with tax. Very attainable for people other than OpenAI and Facebook. And it's not crazy at all to snag a server with enough bays to house all those.
For reference, considering you can purchase a 12-month Spotify Premium subscription via a $99 gift card at the moment, that same $6k could be used for 60 years of Spotify Premium.
For reference, cosidering the backup has 86 million music files, at an average of 3 minutes per file it would take you around 490 years to listen to all the tracks.