How much does it cost to regression test your systems when...
- You need to put a security patch on the OS?
- ...or the application server?
- ...or the database?
- ...or the single-sign on solution?
- When the browser your customers use updates? (BTW, they ALL do auto-updates now)
Most of the mainstream packages now do security patches on at daily or weekly, if not monthly basis.
You can just blow off those security updates (and not be surprised when your data is sold to some scary guys). You could hire a giant team of manual testers to run, and rerun your tests over and over and over again.
Or, perhaps another idea. Automation is really, really cheap, especially compared to security breaches and manual testing.