Check out the following website weekly for sales on LCD screens. I got a 5" screen and mount for under $100. They also have switching boxes to change between cameras. Get an unpowered manual switcher.
http://www.partsexpress.com/
I got a low light B/W camera and a color camera for under $50 each from:
http://www.x10.com/products/cameras_wired.htm
The cameras come with an AC adapter that puts out 12V DC. I just threw away the AC adapter and ran the cameras off the boats 12V DC.
I was even able to hook a camera up to my night vision scope.
Gary