Walmart offers the XO Vision Automotive Digital Media Receiver (pictured), model no. XD103, bundled with the Boss 6.5" 2-Way Rear Car Speakers, model no. CH6520, and a choice of five pairs of front speakers for $46. Choose in-store pickup to dodge the $4.97 shipping fee. That puts most bundles at the lowest total price we could find by at least $2. The MP3 stereo receiver features 80 watts of maximum power, a USB port, SD card slot, FM tuner with 18 presets, and auxiliary port.
Note that other receiver/speaker bundles are selectable, but will cost more.