It can be anywhere from as little as under $100 to $10,000. It really depends on whether the author needs professional services--and what level of professional services.
If you want your book to have covers, interior layout, and editing levels that are similar to something that Random House, Penguin, etc., would put out, then you are looking at between $3,000 - $10,000 (variables include word count, type of cover art, interior layout--lots of charts, tables, etc. add to cost, professional experience levels of editor and book designer, number of formats, etc.). This approach makes a lot more sense for someone who will be using their non-fiction book as a higher level business card (it can even be written off against the existing business' PR expenses until it starts producing revenue).
For a first time fiction author with a limited budget, I would suggest that splashing out on a good cover probably generates more initial sales.