That’s a great question, the fee model can definitely take a small amount of getting used to.
Essentially, the money you make on each sale can be worked out through with the following formula:
Item List Price (The price buyers see on item page) - 20% buyer fee - author platform fee %
In your case, an item sells for $21. A 20% buyer fee is deducted, leaving $16.80 . Out of this, you automatically pay envato a 31.25% (your platform fee), leaving your net earnings as $16.80-31.25% = $11.55
Your platform fee (in this case, 31.25%) reduces as your total sales grow. This handy .pdf gives a nifty breakdown of the percentages: http://d2mdw063ttlqtq.cloudfront.net/resources/rates/envato_author_fee_schedule_(sept_2014).pdf
Happy to answer any questions you might have