The cheapest source of cameras for robots are broken laptops. In most cases the built-in web camera of a broken laptop is functional and can be easily extracted. Furthermore, most laptops use USB cameras. It is therefore just a matter of replacing the connector and using the camera on a robot (see image below).
I have used a camera from a Sony Vaio on a KUKA YouBot rear bumper.
PS: The laptop speakers are also useful for making the robot talk.