Very interesting stats Clay.
$9.44 for a print is pretty low for material costs and printing time. Downright frightening if you looked honestly at depreciation on equipment and photographing time.
On the other hand; given the nature of buyers to browse and peruse on ebay, and accounting for varying tastes, would you consider relisting unsold prints? If they then sold, this would give a very different model. Bit harder to forecast though ...
[Don't forget the number z with probability p lying under the standard normal density curve is called the upper p critical value of the std normal distribution.]