My solution was to buy
this light and a cheap $10 synthetic leather case from eBay. You can get either the style with four loops of elastic/leather in the corners or more protection that covers all but the keyboard, screen, and page advance buttons. Total cost: about $14. Not as stylish, but much cheaper, plus the light can be used with other books. I've read probably 30-40 hours with that light and the batteries haven't noticeable weakened. You can get packs of replacement 2032 size batteries cheap via eBay or similar sites.
One other note: the light I linked to spreads the light more evenly around the screen; I acquired the lighted Kindle case with my used Kindle 3 but ended up selling the Amazon case because of the uneven spread plus the case is still really valuable.