I worked at Circuit City, and we saw the profit margin on every item we sold (and were required to maintain a certain margin percentage in our sales).  I worked in the audio/video department.  The margins people are quoting are way too high.  For stereos, dvd players, tvs, etc, margins ranged from negative to 25% on average.  The higher end items had a greater margin.  The cheaper items had hardly any margin at all.  Speakers had the highest margin, and were as high as 50% on the high end ones, but sales on cheap speakers were often below cost.  Accessories have like a 75% markup.
Our sales screen showed margin in 2 digits.  For negative margins, it would show -5 or -8.  For negative margins greater than 9%, showed **, as there are only 2 digits available.  So we were NEVER to sell "double asterix" items.  That could be anything from -10% or higher, and could kill your margin for the day.
I remember this one time, we had some cheap Pioneer speakers on sale for $29.99 each.  They were ** margin.  Some guy asked what price I would give him if he paid cash.  I said, "$29.99".  Like I care how he pays, and like I have the ability to change the price.  Do you go to the grocery store and ask what the best price they'll offer on a gallon of milk?