The Sennheiser CX400 II typically produces better sound quality compared to the CX300 II due to its improved driver technology and enhanced frequency response. The CX400 II offers a more balanced audio profile with clearer highs and deeper bass, making it suitable for a wider range of Music Genres. If sound quality is your primary concern, the CX400 II is the better choice between the two.
The cheapest pair of headphones Sennheiser make are the Sennheiser CX300-B. They can be bought for approximately $20 from many online retailers and music shops.
Here a few headphones that have gotten high ratings. Sennheiser CX300-B, and Shure E4-n Sound Isolating Earphones. It is really up to personal preference of the person.