To get the edible part of the orange, I use a knife to cut off the peel and most of the pith (illustrated in photos above), then trim out any large pithy or peel-y parts at the core of the orange. The edible fraction is the ratio of the weight of the peeled orange to the weight of the unpeeled orange. Over the last few months, I weighed 23 navel oranges, 4 blood oranges (Moro, I think), 4 Cara-Cara oranges and 2 navel oranges that were labeled "XL" at Berkeley Bowl [update, 2/22/13: all of the oranges were grown in California]. The oranges came from several vendors at the Farmers Market and Berkeley Bowl. It's not a definitive sample, to be sure, but good enough to make some basic (and not very surprising) conclusions.
The figure below shows the results, with the edible fraction (by weight) plotted against the unpeeled weight. The results are all over the place, so the unpeeled weight is not a good metric for finding value in oranges.
Thinking about these measurements as I drafted the post, I realized that I should have also recorded the volume of the oranges so I could check for a correlation between volume and edible fraction. In addition, the right metrics for 'orange value' are probably not the edible fraction, but number of calories, quality of flavor or vitamin C content, items that I'm not equipped to measure (When all you have is a hammer...).
Random link from the archive: The Burrito Dissected