It seems the prep cost of locker items is generally double the price you bought it for in the shop (if it's a shop item). The Tinker buys everything at a discount of 5 gold, so if you use the Tinker to hunt for locker items, your locker preps become cheaper by 10 gold. If the Tinker acquires the item through a Translocation scroll, it doesn't become a free prep but it still gets the Tinker price discount.
Is this supposed to happen? I haven't seen it mentioned anywhere.