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.