re: If an IV bag is just water, why am I paying so much for it?

The bags of "water" given in the hospital aren't just water. It is a sterile saline solution with a similar concentration/tonicity found in your blood stream. 9 grams NaCl mixed in 1 liter of sterile water. Higher and lower concentrations result in fluid shifts sbd electrolyte derangements which can cause death.
Infusing a nonsterile solution into your blood stream is absurd.