wow Matt that's a real neat one. I don't have my porcelain insulator books handy but every porcelain shape has a different number, assigned by collectors. Some are much more rare than others, and like bottles, they are often fairly common in one color (usually a form of brown) and the price can change dramatically for any rarer colors. Although some blues are common, some are very rare. That one you have there has an attractive 'drippy' glaze, probably of older manufacture and adds to desirability like crudeness in a bottle. Although most are unmarked, many collectors know who the manufacturer is by the glaze and shape etc. and can therefore pinpoint a date range on them. In general early porcelain is most valuable. As the more attractively colored glass insulators have gone up in price, more people are turning to porcelain to add some color to their shelves, as they are still fairly affordable even in nice colors.