£100-£200 will get you a cheap cctv standalone system (4channel +) a good quality camera is about £60-£100. The camera can be mounted on a wall, run a cat5 cable from the cctv system to the camera. The 4 pairs are then used to power the camera and for video feed. Pair 1 - VIDEO+. Pair 2 - VIDEO-. PAIR 3 - POWER+. Pair 4 - POWER-. depending on the camera you buy you may have to fit a BALUN, this converts the cat5 to a BNC connector. The same connector will be used on the system end to input the signal to the BNC input on the system. You then connect the power supply at the system end too.
Sound complicated but pretty simple.