A character may recharge an existing staff or wand by using the craft item rules to increase the number of charges in the item.

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.
The character must add at least 10 charges to the item with each recharging.
The character does not receive a discount for recharging the item (the cost per charge for recharging is exactly the same as if she were creating a new charged item of similar type).

While this does not give a discount it is still much less expensive than creating a completely new item.