I read on the European Commission website that if I bring more than €10000 with me when I leave the EU on a flight, then I have to make a declaration at the customs.
In this case, is there any tax or charge that I need to pay? If so, how much?
No. It's merely a reporting requirement, as an anti-money-laundering / drug-trade provision. You will be asked where and how you obtained the money.
The country you are traveling to will likely also ask you to declare the cash, and may ask questions about it.