Cyancryllic via eBay offers the 73-Foot 200-LED Solar Power String Fairy Lights in Warm White for $8.59 with free shipping. (Icicle Direct via Amazon has a similar set for the same price.) That's the best price we could find for a set of similar lights by $3. They take six hours to charge and last up to 10 hours.

Update: The price has increased at Cyancryllic via eBay; however, Icicle Direct via Amazon still offers it for $8.59 with free shipping.