As with most precision equipment, microgram scales are usually very expensive. The close tolerance machining of sapphire pivots and computer control add to the cost. Not all of those components are necessary though. This article from Scientific American demonstrates how to build a simple microgram balance using a salvaged galvanometer. Once calibrated, the output voltage will have a direct linear relationship to the weight. There is a more recent version of this project that uses a microcontroller to overcome static friction and auto center.