on page 549 in corebook it talks about Magi Item Gold Piece Values mainly asking about Item Requires Specific Class or Alignment and Item Requires Specific Skill to Use. Does the 10%-30% reduction go to cost to create hence making an item cheaper if you put a requirement on it or market cost to purchase from a store?
The discount affect both prices (and given their 1:½ relationship it doesn't matter where you apply it first).
But for magic item creation, I'd strongly encourage not allowing this as a PC choice. It is meant as a tool for pricing items with a limited usefulness, but if you can pick a choose their restrictions yourself, it stops being restricting. It shouldn't be used as a free discount on all homemade magic items.