I've got a Safety Vision system, it's black & white and only has one camera and also picks up/plays sound, but the monitor will support a 2nd camera. I bought it on Ebay for $120.... I also have a 2nd Safety Vision B & W monitor that I'm not using, and a Safety Vision color camera that is not compatible with my monitors. That Safety Vision is a high dollar unit that sells new for $300-400...
I had also purchased a basic camera that even though it was a color camera, it did work. But I had some problems with the cable and before I found a new cable I found the safety vision system. I got that on ebay for $10...