The best ones for sound quality aren't made anymore and were never in the $50 range, you're not going to find much for sound quality these days.
- Creative Zen Xtra
- Creative Vision M
- Cowon J3
Pretty much everyone has given up the ghost and just let iPod win due to its interface and popularity. Both Creative and Cowon now try to focus on apps and the other garbage that made the iPod popular instead of going for quality of sound anymore. If you were happy with sound quality from the iPod Nano before, though, then "good sound" shouldn't be on your criteria and there are lots of possibilities with the iPod, of course, being #1 among them.
I believe the Sandisk Sansa Clip/Clip+ is in that price range and has a card slot.