If it's a one time thing, you could consider doing it quickly via a spreadsheet (although I know others would strongly disagree). You could export the data pretty quickly via some sql
INNER JOIN catalog_product_entity_decimal ON catalog_product_entity_decimal.attribute_id = eav_attribute.attribute_id
INNER JOIN catalog_product_entity ON catalog_product_entity_decimal.entity_id = catalog_product_entity.entity_id
WHERE eav_attribute.frontend_label LIKE "%cost%"
This assumes -
- You only have the one store view
- The attribute you're referencing is "cost"
- The attribute you're referencing is a "price" attribute
So you can then export this as a csv, quickly process the calculation, and import it back it, either by an import tool like Magmi (recommended if you don't know what you're doing), or if you have a full grasp of this explanation, you could pop the results straight back in to the database (again, I know for many this is a cardinal sin).
However, always make a backup of your database before you try things like this. You really never can have too many backups.