You want a miligram resolution scale. And preferably one that has grains for a unit. I found several on eBay for close to $300. Metler/Toledo, A&D and Sartorius are all good brands. I have 2 Metlers. Good units.
Side note, I was looking for a scale with the RS 232 interface that was cheaper than the A&D with mg resolution. I was thinking that the communication protocol for all those scales is the same and would all probably work with the Autotrickler. Never pursued it any further but if you think you might upgrade to the Autotrickler setup later, getting a scale with the right interface might save you some money.