While it's shown itself to be pretty inaccurate for rc's I have a scale accurate to the .01 g that I got on eBay for $20 bucks. This is a good 60 dollars cheaper than it would have been normally so I'm sure if you look around you could find some mg accurate scales on eBay for under $100.
EDIT: Here's one: http://cgi.ebay.com/Airgun-Pellet-Digital-Scale-10g-0-001g-154gn-0-1gn-NEW_W0QQitemZ270012343940QQihZ017QQcategoryZ52509QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Don't know how good it is quality wise, but it just shows that you can find some for cheap if you look
EDIT: Here's one: http://cgi.ebay.com/Airgun-Pellet-Digital-Scale-10g-0-001g-154gn-0-1gn-NEW_W0QQitemZ270012343940QQihZ017QQcategoryZ52509QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Don't know how good it is quality wise, but it just shows that you can find some for cheap if you look