Here is the process-
1- Prepare the index yourself with the topics which are needed to be covered.
2- Determine how long you want each chapter to be?
3- Shortlist all the high quality writers on elance, this forum or any other freelancing site.
4- Select atleast 5 Writers or more to do the task.
5- Give one or two chapters to each writer instead of asking them to do the whole ebook.
So let's see how this works...Prepare an index with 10 chapters ie 10 topics...Give those topics out as titles of articles to all the shortlisted writers and give them the word count.
Most writers complete 5 or more articles within 24 hours therefore you can have a complete ebook done within 24 hours by just spreading it across different writers.
Since the writers don't know that they are writing for an ebook they won't charge extra high prices but make sure you let them know that you expect extremely good quality of work.
So in a way you pay less and get work done extremely fast. Let me know what you guys think about this.