A Silvery Mirror costs 20,010 sp (plus XP and time) to create, according to Scrolls of Skelos. But if you ever come across someone willing to sell one, it will likely be more expensive.
A normal silver mirror should not be allowed as a focus for Visions. The real value of the Silvery Mirror seems to be that you don't have to pay 100 sp every time you cast Visions. I personally can't think of any uses for the +2 enhancement bonus, except when rolling opposed checks with Incantation of Fog and Shadow (The Free Companies).