Amazon offers its Prime members the Belkin RockStar Headphone Splitter in Light Green, model no. F8Z274TTP, for $5.49 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, it adds $6.01.) That's the lowest total price we could find by $5. This 3.5mm adapter splits one port into five to allow multiple people to connect to the same music player simultaneously.

Update: The price has increased to $5.79.