You are asking for too much from a scale. If you have to build the scale yourself, you should build two scales; one for up to 600 grams that could easily weigh to the nearest tenth gram and a smaller scale (perhaps 10 grams max) to weigh to the nearest milligram.
If the scale is being used to sell something based solely on its weight, then you have the problem of certifying the scale. If you simply want to use the scale in your process or get a rough idea of material in order to consider when determining an asking price, then you should be fine as long as there are other factors used in considering the price, such as design, labor, etc. but it seems to me that if that is the case you wouldn't need such accuracy. Why is weight to the nearest milligram important?
Dale