The fee is simply the penalty for reusing the funds on a restricted ticket. The new ticket costs what it costs.

Thus:

New Ticket Net Cost = New Ticket Cost + 300 - Value of Old Ticket

Whatever the taxes and fees on the original ticket were are part of the value of the old ticket and you have received those as a credit. Now, you pay the taxes and fees on the new ticket as part of its cost.

Don't think of the credit for the cancelled ticket as anything other than special funds which may only be used for ticket purchases.