The problem with this approach is that it opens up a good angle for your competitors to attack you on compatibility grounds. They can tell your client that the

The problem with this approach is that it opens up a good angle for your competitors to attack you on compatibility grounds. They can tell your client that they don't charge anything extra for IE6 compatibility, leaving you explaining that either they are charging them but that it's a hidden cost, or that their sites are so crappy and outdated that they work fine in IE6 from the get-go.