A pack of cheap clear plastic shower caps and a few rubber bands will proect your camera far better than you'd think it would.. for cheap accessories, it's up there with the Rizla fagpaper flash diffuser...
Alternately, you can head over to sites like warehouse express and marvel at the price of dedicated camera raincovers... don't get me wrong, the 'anorak for your lens' looks very sporting-pro, but I'm not sure it's worth £90..
Or, if you've a long lens, one of those stupid umbrellas-on-a-hairband can be adapted to protect your lens. Maybe £90 isn't so bad, after all...