Check out the Gemini-20. It weighs down to 1 mg and can be found for $21.49 on Amazon, and it comes with calibration weights and everything. I just ordered one myself. I also have a small jewelry scale that weighs to 10 mg which I found for about $15, and it has lasted me over a year now with weekly, sometimes daily use. Keep in mind that for most cheap scales the very last digit isn't very accurate, so for example a scale that weighs to 0.01 g (10 mg) is really only accurate to around 0.1 g (100 mg) or more. The Gemini-20 should be decently (but not perfectly) accurate down to 10 mg. To get an extra digit of accuracy beyond that is going to cost you hundreds for a lab scale, so it's probably not worth it for your purposes.