> I wonder what the costs are though - the quotes are hidden from their site.

You can easy try PVS-Studio. Explanation about the PVS-Studio demo-version limitations: http://www.viva64.com/en/b/0395/

> You cannot simply spend a year fixing bugs no-one actually cares about (because software was working well enough before).

Yes. You can analysis only newly written code. Best Practices of using PVS-Studio. Now with C# support: http://www.viva64.com/en/b/0364/