[TriEmbed] Best way to get several composite video streams into a raspberry pi?

[Brian]

Hey gang,

I recently bought a box truck for my lighting business to schlep my gear
around.  I'm in the process of putting cameras at strategic places 
around the exterior so I can see what I'm about to smash into whilst 
maneuvering.  Rather than have a dozen[0] little LCD monitors scattered 
around the cab, I thought "hey, this might be a cool project for a RasPI 
and OpenGL ES to make a poor-man's bird's-eye-view system like they sell 
to big rigs for $gazillion!"

All the little cheap backup cameras have NTSC composite outputs.  I'm 
looking for the best/cheapest way to get several of those signals into 
the RasPI as, ideally, /dev/videoN devices that I can use as texture 
sources in OpenGL.

It seemed to me that I should be able to find something that has a USB 
plug on one end and a yellow RCA socket on the other for less than, say, 
$20.  There are a ton of capture devices on the market in the $20-30 
range that take in video and stereo audio, but (1) that's just a bit too 
expensive for this project since I'll need one for each camera and (2) I 
don't need audio capture.

Soooo, anybody know of a good, cheap USB composite video capture device 
that might fit the scenario I've described?

Thanks!
-Brian

[0] - A dozen is an exaggeration, but easily at least 4 or 5.