The blockchain has to run on something, so even decentralized it would probably end up something paid. There might also end up a secondary market for domains, just like we have now, but worse.
Also, it most definitely does not cost $10 in compute, storage, or bandwidth to serve your one address. Besides, you basically never pay directly to the registry(the people that run the root and authoritative DNS servers), but instead buy from a registrar (a middleman). A lot of what you pay goes to the middleman, because they can usually get away with asking for it.