This is of interest to me since I am compiling parts right now.
From my research there are 3 choices for balance. You can buy a complete rotating assembling from Scat that is balanced. "BI" is added to the part number. Speaking with them, that means they are about +/- 5 grams in balance of an entire rotating assembly. This is more then likely way better than factory balanced. This usually adds about $200 to the cost.
Second choice is to have your local machine shop balance and skip the Scat balance. In my area with a machine shop I trust their goal is balance to < 1 gram and have balanced Scat stuff in the past and they estimated $250.