The Juice Box may still retail at $50, but kb toys and amazon often have the full media kit (juicebox + mmc adapter) for $15 or less.
Step 1: Gather Materials
1) Mattel Juicebox Media Kit (or juicebox modded to add mmc slot as seen here: http://www.flickr.com/photos/stevenbristol/sets/537129/ mine was 9.99 at kb toys with the media kit (the white box))
2) 350mA 4.5v ac adapter with the correct tip (i used a unit by Coby that cost $3 at Ocean State Job Lots)
3) stereo miniplug to dual rca adapter cable (dollar tree toted these for ipod at, you guessed it, $1)
Step 2: Hook It Up
connect the dual rca to your stereo's aux input
connect the miniplug to the juicebox
plug the adapter in, and check tip and voltage
Step 3: Load Data
So, plug an SD into your computer or card reader and right-click on the drive and choose format
select FAT (not FAT32) as the filesystem and format.
now, load the drive with 128kbps 44100hz mp3 files and enjoy.
I don't normally use 128kbps mp3s, so i'll add an instructable about exporting them from Rhapsody next