I just picked up some random RCA 4GB MP3 player for ~$50 two months back or so. I had no special needs though. I just wanted an MP player for use in my car. There was one that was very similar (only smaller and with a smaller screen) for almost half the price at $30 but with half the space at 2GB. Just pick anything? It honestly doesn't matter too much, and I can say that knowing it's the truth in this case since you didn't list any needs or so forth.
Hmm sometimes they have Creative 16 gig mp3 players for $100 or 8 gigs for $100. Both worth the price because you can more memory later on. Also better video support than a ipod.
Only issue with non apple mp3 players in battery replacement. Your screwed once the battery doesnt charge like it used to. My brother has that issue with his Sansa 16gig that he spent $200 2 years ago.
So maybe best advice spent 50 now save the other $50 for future better mp3 player like a iTouch. I really like the itouch but way out of your price range.
My mp3 player im using myself is $20 dollar 2gig Emerson that is fine by me because i only use it for music. Have a psp for video needs.