It depends what you want to do with the video.
An inexpensive consumer 4-channel surveillance DVR is about $300.
Add in a 500 gig hard drive for about $70 (records the video).
4 cameras - Average $100 each for outdoor rated, night vision, so $400... budget around $1,000 for a low-end, multi-camera, system.
The expensive part is the labor. Are you planning to install it yourself? How will the camera's wires get from the camera to the DVR? Thinking "wireless" cameras? They still need electricity - so is there power where you want to install the cameras?
Actually, "wireless" security cameras use more wires than wired ones... The camera needs power (1 wire) and transmits video wirelessly to a base station. The base station needs power and a wire to the DVR. Total = 3 "wires". A wired camera has one long "combo" cable that carries video and power.
How do you want to monitor that video? The DVR has a video-out connection that can connect to any TV with a working, available, composite (yellow RCA jack) plug. More expensive DVRs can be added to a computer network...
Frys (and Frysdotcom) has a good selection of security and surveillance stuff...