[Duramax 6.6L]

I am trying to find out what file controls what can be entered into the fields that control the price. I wod like to be able to place letters in the item price fields.

I beleive this line controls it, but I can not find the file to edit.

<td class="ProductDetails"><input type="text" name="list_price" size="18" value="{$product.list_price|formatprice|default:$z ero}" /></td>

Where would you find the formatprice controller

Frank