The very bottom of the touch screen on my iPhone does not respond to touch. When the touch screen fails, is is expensive to replace?

Recently, the bottom edge of the touch screen on my iPhone stopped responding to touch. I have gone through all of your hints and tips in the service information pages to see if there was a DIY solution. The little 'dots' which indicate water damage are white. I checked those in case someone else had caused water damage to my phone, as I do not live alone. The home button works fine, the non-functioning area of the screen prevents me from using all functions for which the screen will not revolve to the side when the phone is tipped. Texting is a nightmare, as I have to keep turning the phone to access certain alphnumeric icons on the keyboard. Is this a common problem with the iPhone 4? Is is expensive to repair?