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I have already explained you in the following thread on how to add a calculation value to the options of drop down question: http://www.jotform.com/answers/425495-Questionaire-form-with-Paypal-payment#4
Response copied from the above thread:
Here are the steps you should follow to calculate the price of each drop down options:
1. Add a drop down question in your form and then add the options.
2. Click on the gear icon of the drop down field.
3. Click on the properties.
4. You need to add prices for every drop down options in the "Calculation Values" property. Please check the screenshot below:
5. Add a "Form Calculation" widget in your form and add the formula to read the calculation of drop down field. You can also add the formula in your existing "Form Calculation" widget you already have. Please check the screenshot below:
Hope this helps.
Do get back to us if you have any questions.