In this case $0 or even a negative value amount is what you can plug into something. Why? Think about artists that tried pay-what-you-want model. A lot of people pay $0. Some pay $0.10. But a lot pay $0.
However, what they pay with is with their time and attention.
I used to know someone who managed a movie theater. He would sneak us into movies quite often. Good and bad. And quite often after coming out of the movie I felt like wanting "my free back".
Let's say an artist is unknown and I am a customer that has quite a few connection ( a blog with a large following perhaps ). Then what would make sense is for me to pay a negative amount ( the artist would pay me to spend my time listening to his new cool record in the hopes that I might spread the news about it ).