I've seen $20 a song. I might be able to do that. But is that the usual rate? I dunno.
Let's give this some thought.
You're talking about original music, so let's say someone writes 10 tracks for your game.
They're exclusively licensed to you and tailored to your needs. The composer gets 200 bucks.
Now how many jobs like this could anyone do in a month? And do you think anyone could make a living with this payment model? Not even considering the composer's education or running costs for studio rent, equipment, maybe hiring instrumentalists, buying licenses to sample instruments...
Wherever you saw this 20 USD offer, it'll be neither original nor exclusive music.
Or maybe it's a start-up 'professional' doing some serious price dumping, happily digging his own grave.
Bottom line:
Get a hobbyist to write music for you. Someone who's doing it just for kicks - if that's what you're doing too with your game dev.
If you're aiming for commercial success, you'd better have a budget or a really good idea and some progress to show or someone who's sharing your vision without any compromise. Preferably, all of that!
Cheers,
Moritz