How can I have a field value that is the sum of other (decimal) field values?
This question is tagged with Views, so I'm just going to go out on a limb and leave this here just in case: Views Calc


  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).