A good copywriter will usually charge $10-$15K plus a percentage of sales.
That's rubbish. I write copy for clients all day; very few companies I work with launch without copy for websites and marketing materials written by me. I bill this at my standard daily billing rate. I am without doubt a good copywriter although it's not what goes on my business card; it's just part of what I do for clients so that they launch as well as possible. The pro copywriters I know may be charging 10x what I'm charging but it's still nothing like what you're talking about.