Funny you should ask, I just got a cheap on on eBay because I have an extra 1GB SD card. I won with the minimum bid, shipping is bloated but S/H + insurance totaled $21.
Check it out:
http://cgi.ebay.com/SD-MMC-Card-MP3-...QQcmdZViewItem
Should you decide to get this cheapie, it only plays MP3, no ogg, wma. Don't rip above 192kb, highest I've tested is around 220kb and sounded all right. One last thing, it reads up to 2GB although the description says 1GB.