Ttstoreusa via eBay offers this Motion-Activated LED Toilet Light for $4.66 plus $1.39 for shipping. That's the lowest total price we could find by $2, excluding other eBay vendors. It features red and green LEDs, motion- and light-sensing technology, and a 30- to 50-second automatic shut-off. The light comes with adhesive mounting strips and requires two AA batteries, not included.
Note: This item ships from mainland China and may require two to four weeks for delivery.
Update: As a reader found, antiquechina2009 via eBay offers it for $4.82 with free shipping.