I did say marginal cost...the cost of producing one more ebook. Which is very very close to zero.
There are lots of costs currently involved in making a book as you so rightly say. But currently the industry enforces this on all authors because the costs of printing are so large that it's worthwhile hiring an editor, typesetter, proofreader, etc to ensure that the book sells more.
However, if the book is going to be purely released in ebook format, the cost of publishing is zero, so all of those editing, typesetting, marketing costs are entirely optional. If your book contains errors but you can drop the price to $5 because you don't have to pay a proofreader, will that make you more money? Will more people buy your book because it's only $5 than are put off buying it by the bad reviews because of the errors?
You could even produce two versions of the book, one at $5 with errors, one at $10 with no errors, the electronic equivalent of paperback and hardback maybe? There's lots of other, different models that might work for ebooks but don't work for paper books. If the publishing houses don't get this then they will have it got for them.
This world is already here, as anyone who has perused Amazon's ebook market will know. Cost of publishing = zero, so endless cut-and-paste copies of Gutenberg originals being sold for 99 cents. And as the music industry is proving, once you have a reliable online marketplace physical sales dry up. See Borders for details. Brick-and-mortar bookshops are struggling, Amazon is booming.
It'll be interesting to see how the quality part of the market survives, but there'll always be a demand for a well-written book so it should settle out in the end.