this isn't much of an instructable, as the components can be purchased and combined, but i don't know how often people have done it before. I'm using a Mattel Juice Box as the mp3 player for my stereo system.
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
Here are the necessary 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
I said this wasn't much of an instructable...
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
The juicebox takes sd cards up to 512mb in size, but they have to be FAT16.
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