There is no difference in the media you use WRT sound quality. The 5 cent off-brand Chinese CD-Rs sound exactly the same as the one dollar Sony or Maxell discs.

The difference lies in stability. Sometimes the cheap discs don't play in some players. Sometimes they skip or "tick" on playback because the recording layer is too thin and the player can't read the disc in real time, so it hesitates.

But if you are using the exact same source material on the recordings I defy ANYONE to tell the difference in a blind A/B test between cheap and expensive discs.

Either you are accidentally burning your discs as low end mp3s or you are only imagining this perceived lowering of fidelity. To prevent the former, go to iTunes and the View> View Options and activate the "kind" checkbox and it will show you what kind of file you are working with. Also, use Preferences> Advanced> Burning to make sure you are burning in the right format.