Followup on Vladinator's DepositBox2 that was up in Patch 4.X, and SkOODaT's addon "DepositBox" that was up to patch 3.X.
Updated to work with MOP 5.3, now I did use a lot of the code from DepositBox2 (my first time ding this so please bear with me)
When you login you are told how much the tax is and how much you have specifically stored away. Try it yourself, use /dbox /dbox2 or /deposit to manage the addon in-game.
Note: the "tax" works like this, from your actual gold amount it first subtracts the gold you specifically save and then it simply removes the % you specify from that total amount. The leftovers are put as your actual gold in your bags.
I also made it so you can't buy items you can't afford. The only way to "disable" the addon is to set taxes to 0 and use the "tak" command to take all money that you have deposited/reserved.
Data Broker: you can toggle the data broker label by holding down either ALT, CTRL or SHIFT while clicking the text on the broker addon. You can not do this in-combat (made it so you can't toggle this by mistakes). The tooltip shows the expected information so you do not have to /dbox to find out the gold status every time.