Run the built-in self test on the screen - hold D through powerup.

If you get a good set of test screens, it's likely the GPU is bad and the mainboard needs replacement.

If the built-in self test shows the same fault you're seeing, the display assembly needs to be replaced.

A display assembly will run $200-300 plus labor; a mainboard, $300-500+ plus labor.

Dell out of warranty support:

http://www.dell.com/support/article/us/en/04/sln284409/how-to-arrange-out-of-warranty-repair-on-your-dell-system?lang=en