San Pellegrino Sparkling Mineral Water - 750ml glass bottle screw cap for $19.95 a case.
Combine with AMEX offer (spend $50 get $10 back) and Cashrewards. Three cases will end up costing you just over $15 each.
Now feel free to enter "water is free" comments
I just bought a soda stream as I was getting sick of carrying bottles of water back from the supermarket.
So yes, tap water is cheaper, but I also like it carbonated water with a meal.