Hello, Heidi! 
I'm so happy you're coming to the Disneyland Resort soon! It's a joy to help you today. Thanks for getting in touch with our planDisney family. 
The cost of extending your 
ticket will depend on the date you're planning to visit. You'll be able to make this ticket modification in person at the Disneyland Resort, and the Cast Members will be able to inform you of the associated costs. 
Since you really want to make the most of this trip, I advise you to talk to a Cast Member at 
Guest Relations as soon as you enter a theme park on your first day. If you're going to 
Disneyland Park, you'll go to City Hall on Main Street, U.S.A. If you're going to 
Disney California Adventure Park, go to the Chamber of Commerce on Buena Vista Street. 
I hope you have a blast at the Disneyland Resort, Heidi! 
May the magic be with you, 
Julie