Hmm. If you're buying something cheap and new, I think you're likely to be disappointed by the quality, particularly in low light (caving) conditions.
One thing to consider is to do video recording with a digital still camera.
I've been fairly impressed by the quality of movie recording on a Canon Powershot A520, and its probably fairly cheap to pick up similar models as a 'mere' 2,3,4 megapixels is not considered enough anymore for stills.