According to Wikipedia, the most expensive item by weight and volume is the Treskilling Yellow stamp from Sweden. It has a current estimated worth of $2.3 million. Here's what makes it so valuable: In 1858, when the currency was known as the skilling, the 3-skilling stamp ("treskilling") was printed in blue. And an 8-skilling stamp was printed in yellow. But due to a printing error, a few 3-skilling stamps were printed in yellow.