Recharging Spell-Trigger Items:
Divide the item's market price by 50 to determine the price per charge.
Multiply the price per charge by the number of charges the character is adding to determine the total cost of improvements for the item.
Determine time, gp, and XP expenditure normally based on
the improvements cost.
While this does not give a discount it is still much less expensive than creating a completely new item.