SoundCloud could easily exist just by asking artists that want to host more than 5 songs a fee, or for some extra features on their profile. Just small stuff. If they would be dealing just with sound instead of sound-that-is-music.

Serving a service like SoundCloud isn't technically challenging or expensive. It's dealing with all of the legal stuff of people uploading music that's under license, people uploading songs that have uncleared samples in them and all that bullshit.

It's proabably ran by 5 engineers working on the service itself, 25 maintaining the DMCA platform and 270 lawyers.