cy_monsters via eBay offers this 100-LED Light String in four colors for $4.69 with free shipping. That's tied with last week's mention and the lowest total price we could find by $2. They're waterproof and feature several different lighting modes, color settings, and patterns.
A close price: xihusz2012 via eBay has them for pennies more.
Note: Expect shipping to take up to three weeks.