For scheduled replenish, free of charge, but

if for whatever reason you want to have dealer replenish outside the schedule, then it's pretty expensive (more than $100). If DIY, maybe about $10 for the fluid from what I read and should be easy once you know how (check out the link in the top). In case of DEF mechanical breakdown, then everything with BMW is either free (under warranty) or really expensive (out of warranty).