Paying $16.50 a month for unlimited songs is just amazing.
You pay this only once a month, not every time your video/advertisement is shown. If you compare that with a single song on iTunes for example, that costs $0.99, maybe it seems expensive, but this comparison has no sense. While that song is just for your own listening, the Royalty-Free song is going to be used in another end product that can reach millions of people and (even) make money.
So, Royalty-Free music is not an end product, just a piece of another bigger one.
How much business software costs? Or footage? Or a premium WordPress theme?
You can compare the license prices to those products.
Even the most expensive license or subscription is really cheap when you think about it: you can use the music in a TV advertisement or a big movie!… while hiring a composer or a band will have cost you thousands of dollars.
Prices of royalty-free music are low because of scalability: the Internet has allowed the musicians to store and distribute easily any song to thousands of potential buyers and sell many licenses for a single unit (song).
If everyone knew the work that’s behind it, they will say that is really cheap ;)