That discount system is for DM-generated treasure. It reflects that items which aren't as easy to use should have less of an impact on WBL. I usually allow players a 10% discount if they make every item they own usable by them only, though. Mainly because it makes the items worthless to anyone else, and keeps WBL from blowing out of proportion with character death.

But yeah, that's a DM tool, not a player tool. Hence, why magic item creation rules are all in the DMG.