First you need to buy or find the materials. All are avaliable on Amazon.com.
1 x automotive backup camera (avaliable at http://www.amazon.com/Koolertron-License-Plate-Backup-Camera/dp/B0049LBJ56/ref=sr_1_1?s=automotive&ie=UTF8&qid=1338242181&sr=1-1
1 x automotive backup monitor (avaliable at http://www.amazon.com/3-5-Inch-TFT-Monitor-Automobile/dp/B0045IIZKU/ref=pd_bxgy_e_img_b
1 x 9.6V RC car battery (avaliable at http://www.amazon.com/Tenergy-2000mAh-NT8S600B-Transmiter-Airplanes/dp/B00384JI3K/ref=sr_1_13?s=toys-and-games&ie=UTF8&qid=1338242492&sr=1-13
1 x box, I used 6-1/2" x 5" x 3", but anything that fits all the components will work. I made mine from leftover acrylic spray painted black, so cost is neligible.
1 x power switch, avaliable on amazon, or radioshack. $0.5
1 x IR led array (avaliable at http://www.amazon.com/Board-Plate-CCTV-Security-Camera/dp/B0057DPXI4/ref=sr_1_11?s=electronics&ie=UTF8&qid=1338242899&sr=1-11
total price: 49.65