A lot depends on start or BIN price once you are paying for non store listings, it is only 50c for items under $20, from $20 to $99.99 it is $1.50 and $3.00 for over that amount.

The OP has been selling pretty consistently over the last year but the question is of course how many of the 30 free listings were not working because he would easily be able to cover the insertion fees for the sold items.