Showcase doesn't allow prices of existing vouchers to be changed once a item has been sold in order to preserve the integrity of the financial reports.
However, you can copy an existing voucher, change the price, and then disable the old voucher.
As an alternative method to the above:
<right-click> on the voucher
select Change Price
change the existing price of the voucher
select if you want to disable the existing voucher
Note:
By disabling the original voucher you will also disable any member bonuses that have this voucher linked as part of a member reward
See also: