The Talktalk website hack, and subsequent breach of its customers' data, has cost Talktalk £35 million and counting. There's also it's brand reputation to consider.
However, there's plenty of good advice around to help. Here's a very good article about some simple, practical steps you can take to make it much, much harder for the hackers to get in.
The Institute of Directors have also published a good article on the subject which looks at some of the broader, commercial considerations.