Friend and I have recently launched a side project where we give away high quality, royalty-free music. Early days but we’re making ~$800 per month from YouTube’s content ID payments alone.
It is royalty free if they stump up the up front cost, but the idea that using this music will cause you to lose ad revenue on your YouTube videos should be front and centre.
Many of the people using our tracks are small YouTubers that aren't even eligible for monetisation. The bigger YouTubers who want to monetise are happy to buy a $39 license.