Zen cart 1.5.1

Can you help?

Here's what I am trying to do...

I need to record the product item's cost price after checkout so I can create a single sales export which includes the final price and the cost price. The cost price is to be recorded in the PRODUCTS table in a new field (cost), each time a product is added, along with the usual info.

I have also created a new field in table ORDERS_PRODUCTS called my_cost_price for the purpose of recording the cost price for a sold product, but how do I get that field populated when an order occurs?

I think this is might be done via includes/classes/order.php and even assuming that is correct I am not at all clear how to call up the data from the table PRODUCTS for the products_model that has been sold - so that it is inserted into that table.

Is it possible - if so how? Any possible issues about doing this?

Thanks.