Add javascript to get the value of checkbox

Thanks for reading my question.

I am having a specific requirements where I am selling three tickets type using contact form 7.

At the top the checkbox is presented to user where the user can select the ticket type he wants to purchase:

Ticket Category1: 30
Ticket Category2: 20
Ticket Category3: 15
When the user checks each of the category we have a text box that shows the total cost. My question is simple how to achieve this in JavaScript so that total of all three or whatever user chooses will get into the total amount text box.

Thanks
ST