You pay less per day the longer you stay at Disney World based on how they set up the pricing of their tickets. Once you purchase 5 days, the cost per day drops considerably.
Here is a breakdown of how much it will cost you upgrade your ticket. You can either upgrade it before you arrive or you can do it at the ticket window. Just make sure you have one unused day on your ticket or they will not let you add more days to it.
Upgrade a 5 Day Ticket to a 6 Day Ticket for $11 + tax
Upgrade a 6 Day Ticket to a 7 Day Ticket for $9 + tax
Upgrade a 7 Day Ticket to a 8 Day Ticket for $5 + tax
Upgrade a 8 Day Ticket to a 9 Day Ticket for $5 + tax
Upgrade a 9 Day Ticket to a 10 Day Ticket for $4 + tax