Amazon offers its Prime members the Boss Audio 2.5" 400-watt Weatherproof Motorcycle/ATV Amplified Speaker System in Black, model no. MC300, for $34.99 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $5.99.) That's the lowest total price we could find by $3. They feature a frequency response of 20Hz to 20kHz and mounting brackets for handle bars.