How to calculate the sum for each option of all webform results in a Drupal webform grid?
Create a 'webform submission' view. By using the Views Calc module you can display the sum of each field at bottom of the table/grid.

This module adds simple calculations to a views table. Requires the Views module.
You can SUM, AVERAGE, COUNT, MIN, MAX, or AVG selected columns. A plug in theme makes the view look more spreadsheet-like (right justifies numeric fields, shades calculated columns and rows, and underlines calculated rows).

Image from module page