A 159

Since there is no minimum cost listed for the powdered silver, I'd say an arcane caster with the eschew materials feat would not need it.

A 160

That depends what you mean by dropping a shield and what character is dropping it:
Quote Originally Posted by SRD
Strapping a shield to your arm to gain its shield bonus to your AC, or unstrapping and dropping a shield so you can use your shield hand for another purpose, requires a move action. If you have a base attack bonus of +1 or higher, you can ready or loose a shield as a free action combined with a regular move.

Dropping a carried (but not worn) shield is a free action.