As a regular seller, I am sure you are aware of the following, but I restate it for the record..
There are a many added fees for additional services which, IF you use them can add up very rapidly (ie: bold, picture paks, highlighted, additional country site, etc.).
On a lower priced item, these fees can vastly increase your cost percentage. These fees are less likely to be as obvious nor skew your percentages on a higher priced sale.
Another factor to consider is the cost of listing and relisting items that do not sell the first time around.
It's very easy to jack your costs up with little or no appreciative added value to your gross sale. The fees only serve to cut into your net profit and/or increase your expenses.