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If you need to perform calculations using the Ticket Purchases widget then you need to split the widget in single widgets, one per option you need to sell.
You will need to add the price in the widget's option:
Then you need to add the Form Calculation widget to your form:
Add the single widgets in the calculation to get a final result:
And finally, this guide will help you to pass the calculation widget's value to the payment field: https://www.jotform.com/help/275-How-to-Pass-a-Calculation-to-a-Payment-Field
Hope this helps.