Weight is a better gauge of value. Think about this example, 1 2x4 brick or 3 2x4 plates stacked. Should the brick be $.10 and the 3 plates $.30, or should they be about the same price?
Having said that, I do still look at the price per piece, after I subtract $3 per minifig when determining a good value. Large molded figs like dinosaurs, Jabba, Wampa, etc complicate things. Thats why the Dino sets and Jabbas Palace feel way over-priced.