I'm not quite sure I agree-- most guilds don't require anything like this. The biggest costs I can think of simply involve raiding repairs, and not only is not every member of every guild involved with raiding, but repair costs aren't that big a deal anyway, especially with daily quests throwing out money for just a few minutes' work. There may be costs coming in the game (guild housing would obviously require a lot of money, and we still haven't been told how siege weapons might work in guild battlegrounds-- will we have to buy those?), but at this point, we don't really need guild taxes or membership fees.
The closest thing my guild ever did to a membership tax is that they ask everyone to pay a few g to any engineer that drops a repair bot during a raid (to cover those costs), but considering how the new instances are set up, I haven't seen a repair bot need to be dropped for a long time anyway. Does your guild need enough money to require membership fees or taxes? And if so, for what? Potions?