Someone entering your system, which is supposed to be secure, and changing the headline on your front page mean that you'll (if you're sensible) then go and contract an external firm to perform a security audit and find out how the intruders got into your system, what needs to be done to fix it, and why your system had that particular vulnerability to begin with. All of which costs money.
Alternatively, you'll use your own staff to do the same - which isn't free either, since they're now not doing their normal jobs for the duration. If I'm paying you $120,000 to be my sysadmin and then need to pull you off your normal duties to investigate someone entering my systems for a month then that investigation has cost me $10,000. That I was going to be paying you that money anyway isn't really relevant - that was the cost of your time that was used on that investigation.