Now my Nexus dock costs 10% of my battery while using and I wonder whether it takes much battery or not. Anyone know how to reduce Nexus's battery cost? Thank in advanced.

Nexus is a graphics intensive application and all those nice visual effects can consume a lot of your CPU when in use.

This said, it does feature a very extensive and flexible battery saving option which might help (bearing in mind that you cannot have your pie and eat it too, one has to make choices): please check out the Power Saving Mode internal command.