We hear this question from time to time.
The answer: a whole lot.
For example, attackers can copy your customer data. This happened in 2018 to the children's chat portal knuddels.de – and some of the data was stored unencrypted. Such access must be reported, and the data protection authority will then take action, imposing penalties of several tens of thousands of euros.
In the case of knuddels.de, the fine was €20,000, which was low because the company cooperated. Together with improvements to IT security measures, the costs ran into six figures.
Your clients must also be informed about what happened to their data. If credit card or account details have been compromised, they must have the opportunity to take action. This will inevitably cause fear, worry, and annoyance. If damage has been caused, they also have the option of claiming compensation.
Apart from the significant costs, a data leak also costs you something else: the trust of your clients. Nobody wants their address, account number, credit card details, and name distributed online. It's bad enough when it happens, but imagine if it comes out that the leak could have been avoided, that the data was stored unsecurely, and that the whole mess was actually your fault. Now you have clients who will rightly never say a good word about you again – and who may also vent their displeasure online.