Lucky Electronics via Amazon offers this 12- to 24-volt Triple-Socket Car Charger in Black for $3.30 with free shipping. Outside of the price below, that's $2 under the lowest total price we could find for such an adapter. It converts a single 12-volt car cigarette lighter outlet into three.

Of note, this item ships from Hong Kong and may take several weeks to arrive.

A close price: eforcity via eBay charges $3.84 with free shipping.