There are two ways to do it. Add the field to the collection and get the data from the database, or calculate it in PHP based on the 3 values returned from the DB. Doing the first way with the Magento Collection would, in my opinion, be too complex. instead, you want to use a Renderer (Mage_Adminhtml_Block_Widget_Grid_Column_Renderer_Abstract)

First, inside of the Block/Adminhtml folder of your plugin, make a new folder called Renderer. Inside of it make a new file called CostSum.php with the following contents:

<?php
class Company_Module_Block_Adminhtml_Renderer_CostSum extends Mage_Adminhtml_Block_Widget_Grid_Column_Renderer_Abstract
{
    public function render(Varien_Object $row)
    {
         return $row->getStampCost() + $row->getInkCost() + $row->getFormCost();
    }
}

Then, in the grid, make a new column

$this->addColumn(‘cost_total’, array(
    ‘header’    => Mage::helper(‘imprint’)->__(‘Stamp Cost’),
    //’index’     => ‘Im not sure this is necessary’,
    ‘type’      => ‘price’,
    ‘currency_code’ => $store->getBaseCurrency()->getCode(),
    ‘renderer’ => new Company_Module_Block_Adminhtml_Renderer_CostSum()
));