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.
That depends what you mean by dropping a shield and what character is dropping it:
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.