For some simple brochure sites, it's 4-6 hrs more of testing & tweaking during the lifecycle of the project, which usually translates to $500ish extra. i.e. 10-25% of the project.

For involved sites, it just depends what is being done and whether you're trying to replicate the experience or if you're just creating a graceful degradation for IE6 users, which is often "good enough" for a lot of clients.

For basic "looks the same" stuff this is probably true. The difficulty comes from clients who devise features that only work well on newer browsers, then insist on keeping the same functionality rather than "degrade gracefully" on IE6.