Cost=15 bucks give or take based on what you may have lying around
Things you will need:
Strip of SMD 5050 LEDs, I used warm white like these
12 V power supply like this http://www.ebay.com/itm/320766833654?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
Wire sturdy enough to hold up small cardboard(I used 22 AWG radio shack wires I had previously for a headphone amp project)
Old cardboard box
soldering gun/solder/safety goggle/blah blah
Step 1: Prep the Materials
I cut out 3 rectangles from an old cardboard mailing box as the mount for the LEDs. I covered the whole rectangle in electrical tape so it was nice and black.
Cut your wires to size based on your needs. Keep in mind the wires are also the supports so you need at least an extra few inches for that. For my purposes, I had 2 long wires for each light, and 2 wires (one pos, one neg) to connect all 3 to the power supply. I used a push pin to poke 2 holes on one side so feed the wires through so it can reach the LED and bend it flush. Strip a little bit off all ends.
You can stick the LED on with the built on adhesive backing.
Cut off the round end of the cell phone charger, cut open a bit of the overlying sheath to seperate out red and black (red pos). Sheath a bit off the tips.
Step 2: Time for the Hookup
As in the diagram, after soldering the wires to the LEDs, I joined all 3 negative wires with one output negative wire. Then the positive wires were also soldered together with one loose input wire.
You can then solder the loose single pos and neg wires to the power supply. Test it out.
Step 3: Mount It!
Plug it in and enjoy.
You have a lightweight, cheap, energy efficient picture light using about 1 watt!