Andorsales via Amazon offers Prime members the Dell AC511 Stereo USB Soundbar, model no. NCW95, for $21.37 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $8.90.) That's a buck under last week's mention and the lowest total price we could find by $2, outside of the mention below. This soundbar is fully USB powered and requires no external power supply or batteries.
A close price: Dell Home has it for $22.49 with free shipping after coupon code "EXTRA10".