Well not really disposable maybe but I've been dreaming and looking for a <$50 projector for a while now, something I can use when the projecotor is at great risk of damage or theft.   (and small battery powerd would be cool too)
Quality of course is assumed to be at an absolute minimum.

I've been watching the zoom box since it's got the dvd player built in, but it's still priced too high. 

My best option I thought was to hack together a Opque projector and a portable flash drive, portable DVD player with a screen, or a old PDA or digital camera that can play video.  But the price has been a little over my $50 target and the work to get it to work right and dependably is enogh to make me wait untill I really need it.

But today I spoted the Senario AL-100 Mega Vu Torpedo 6ft Video Projector at buy.com for $70 which is still over my target, but the closest I've seen.  Crappy quality, and you will still need a video out device which would bring the cost up to $95.  But it's pretty cheap, and could be run off battery fairly easily.

I'd pull out an old digital camera to feed it images, (I have two semi-broken cannons that can play video, though I can't figure out how to encode a video that they will accept, so I'd have to shoot it off screen) or pick up one of these Coby DVD-209 Ultra Compact DVD Player.

But if $95 is still too much my old plan was to try something like a couple of these hooked together.

Porta-Trace / Gagne Mini-Sketch Opaque Projector $30
Tracer Jr Projector $30
Samsung DVD L25 Mini DVD player $24
philips gogear media player $35