Prices are based on server as well as driven by players. For Yin, BE prices are about 35kk (35 mill) a stack. Now prices go up and down depending on demand as well as who you're buying from. Redfox doesn't set any ingame prices....it's a player driven economy.
In Tao I wouldn't pay more than 25kk per stack of BE. Just search auction house, Tao always have many stacks of BE in there for 20kk.