Hi ron23, and welcome to cruise-chat;
The price varies by cruise line but is between $2.50 and $3.50 a bottle. The soda card is a good deal if you drink a lot of soda, but it does not include bottled water.
There has been a lot of debate on the subject, but many of us, myself included, believe the ship's tap water is actually cleaner and tastes better than the bottled water. Of course you may take your own bottled water with you onboard.