LightInTheBox offers the Ouku 720p Wireless IP Home Security Camera for $18.49 with free shipping. That's $12 off list and the lowest price we could find. It features 2-way audio, motion detection, night vision, and 802.11g wireless.
Note: This item ships from China and may take two to four weeks to arrive. Uncheck "Add Shipping Insurance to your order" during checkout to remove the 49-cent fee.