How? It may need little CPU or bandwidth, but at the end of the day it's hardware, it needs to be somewhere, be powered and cooled, etc etc. The cost may be low enough to be paid by the owner personally, but it's not going to be $0.
Like someone else said, the load is low enough to run on dirt cheap shared hosting like Dreamhost. So even if the cost isn't zero, it's less than what most people spend on snacks.
