The RS case is very nice, though a bit dull...
I'll post pics later of my case, which is basically a small sandwich box with holes cut out.
However, if you want a REALLY cheap option, have a search around for paper/card cases that you can print yourself.
The Adelt case is _very_ good, though the HDMI slot is in slightly the wrong place (they know) so you might want to wait for the next version. I beta-tested the Adelt one. Doesn't require glue, very sturdy plastic (took a lot of abuse as I assembled and re-assembled a lot) and fits the Pi snugly. It's made of the kind of plastic used on overhead projectors, so it's not a 'solid' plastic case, but if you want something to just make it a little more stable, it will suit your purposes well.
My Raspberry Pi blog with all my latest projects and links to articles
http://raspberrypipod.blogspot.com. +++ Current project: PiPodTricorder - lots of sensors, lots of mini-displays, breadboarding, bit of programming.