Each guild will have access to six tabs, each with 98 slots of available storage space. The first tab will cost 1g to purchase, with the costs rising exponentially until you reach the sixth tab, which will cost you 10,000g. Doing the math, Niko figured out that it you will have to spend 16, 111g for all 588 slots currently available on the system as it stands on the PTR. Access seems a bit tricky. It's either a you can remove items or you can't kind of system, which means you really need to have faith in those with access. Those who do have access can also use guild funds to repair their equipment should they need to do so.
Does this seem a bit expensive to you? I know it does to me. On one hand I know that with my bank alt I was full rather quickly, so all those slots will be most welcome. But I don't see anyone other than officers in my guild ponying up the cash for bank slots. So the guild banks might end up being a tough sell to the smaller guilds. I'm just not sure yet. We'll have to see.