@nicolas - totally agree
@Symbiot - in theory yes, but I am very new to HikaShop and extremely busy learning as I go configuring for urgent project! Perhaps there is someone out there with more time...
As a first step, you need to at least create the custom field in the product table - make sure you limit display to 'back-end' (admin) only. You will then see this in the Product add/edit admin forms and you can then start entering your cost prices.
I personally would't attempt to code the reporting in to HikaShop - this may be a good feature for the 'wishlist'!
I would just use PHPMyAdmin or other SQL tools to extract the data and create reports.
If I get some time to dig deeper into this I will post my methods.