If one had no money to buy and no other source, he would soon die. I would imagine that the law of supply and demand dictated that this water sold for a high price. It appears that the natural water being sold didn't really satisfy. People were paying their hard earned money for something that will not quench their thirst. An appeal is made to simply hearken to the voice of the Lord and receive spiritual Water that do satisfy.
Price is no object.
A higher price than we could ever pay, has already been paid. Jesus bought us the Waters of Life with His Own blood. When we incline our ear, God covenants with us. That sounds like a pretty solid covenant to me. Jesus was attending The Feast of Tabernacles.