$40 may have been an exaggeration. Here's one for $48, though. You have an exaggerated sense of the cost this would add. It isn't 2006 any more.
Even if the drive costs only $30 you still have to add hardware margins, warranty costs, integration costs and retailer margins on the final product. It is a major cost for a console and hence if there are alternatives they need to be explored. It starts to get expensive having both a HDD and an optical drive in the same console and you essentially have two devices duplicating each others functions because neither quite does it all.