Cash Stash is a simple addon that tracks the combined amount of money on all your toons on your account. Some other addons have this included, but Cash Stash is designed to be clean and small.
Once you install Cash Stash, make sure that it is enabled for all your characters. You must log into each character once before Cash Stash will track their money. After that, you're all set. Open your character panel to see the total amount of money you have stashed away next to your portrait.
Whenever you gain or lose money on any character, Cash Stash will track it and display the information on all your characters. Note that this will not track any money sitting in your mailbox. You have to open the mail and take the money before it registers.
If you click on the money displayed in your Character frame, a menu will appear that lists how much money is located on each toon. Great if you need to dip into someone's pockets to buy a new mount, but forgot where your money is saved up!
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Important note: If you delete a character that has money (I don't know why you wouldn't mail the money to another toon first, but...), Cash Stash will still think the character exists and won't display the correct amount of money. You will notice that their name is still on the list of characters in the menu that appears when you click on the stash.
To fix this, type in game:
/run CashStashBank["Playername(Realmname)"] = nil
For example, /run CashStashBank["Recompense(Uldum)"] = nil
You should see the information be removed from the chart.