While the specifics relating to cost havent quite been nailed down yet, the system works like this. You pay to turn on Void Storage by visiting a Vaultkeeper in Stormwind or Orgrimmar and get 80 slots for your gold. To remind you that this isnt your bank account, more of a storage unit that you might only use once every so often, you have to pay to deposit and withdraw stuff. Anything you do put in loses its gems, enchants and reforging and is reset back to how it was when you first equipped it and there will be item restrictions on what you can actually deposit.
Using Void Storage is also really simple, you just open it up, drag your items into the storage section from your bags, pay your money and let them start gathering dust. That said, Blizzard has also added a handy search function so you can easily find those items. This is great if youre anything like me, I can literally spend 30 minutes trying to find something in my bank only to realise the icon is staring me in the face. Never again! Huzzah. This function works across your bank, bags and the new Void Storage and its another sign Blizzard is really listening to players and trying to make 4.3 the most awesome patch ever. The company has promised more previews over coming months so were in for fun times until BlizzCon.