cost of supporting old browsers

I've begun to question the wisdom of standards for websites that require support for older browsers like Netscape 4. Backwards compatibility is all well and good, but I feel like often times clients--government ones in particular--forumulate these requirements without any real cost-benefit analysis.

Keeping support for Netscape 4, IE 3, etc. has a very real cost, with respect to CSS and DHTML/Javascript one-offs, or having to pass on new patterns like AJAX. But many statistics show that fewer than 1% of all browsers are Netscape 4--even going back a year or two. (See here for an example.) Most people are using IE 6.

On the other hand, a commercial client I visited had done a very careful cost-benefit analysis on operating system support--they knew exactly how many of their users were still using Windows 98 and how much it would tick them off if they were forced to upgrade. So they might be stuck supporting it, but at least they could justify the decision.

So, before making requirements to support Netscape 4, clients should do their homework, and ask themselves what the benefit is--is anyone really still using it and can't upgrade? And how much extra time and money are the developers going to spend on workarounds to support these old browsers, compared to the value delivered?