A commonly recommended good MP3 player around $30 is the AGPTEK A02.

• The AGPTEK A02 is highlighted as one of the best cheap MP3 players, praised for being very inexpensive, simple to use, and offering excellent battery life (around 70 hours) for basic music playback.1
• It supports microSD cards (so you can add plenty of storage cheaply) and is designed as a straightforward, no-frills player that “earns its spot because it’s fun to use, has a retro appeal, and is cheap.”1

If you tell me what matters most (Bluetooth, clip for workouts, storage size, radio, etc.), I can suggest a more tailored option in that same price range.