The Undefeated Campaign uses the Improving Magic Items rules located on page 233 of the Magic Item Compendium with the following exception: when combining two magic items, the cost for doing so will be as follows. When combining magic items (excluding bonuses found on table 6-11) the cost for each power (magic item) will be x1.5. Example: Narry already owns the Dimension Stride Boots which are worth 2,000gp. He wants to add the Slippers of Spider Climbing item, which costs 4,800gp, to these existing boots. He must pay an additional 1,000gp for the Dimension Stride Boots power (x0.5) and 7,200gp (x1.5) for the Slippers of Spider Climbing power. Now his boots have a market value of 10,200gp and possess the powers of both items.
Clarification: If you have an item with bonuses from table 6-11 on it already but no “unique” item powers you can add power of one item to it, as long as the power of the item you wish to add are for that body slot, for the market price of the item (not x1.5). Example: Narry has a circlet with +2 INT on it, he wants to add the power of the circlet of the magi to it. He pays the market price for the circlet of the magi and now he has a circlet of the magi w/+2 INT.