Richard, I don't charge by the word, but offer a flat rate for book work. For example, I wrote one roughly 40,000 word book through Upwork for a flat fee of $6,000. More than 95% of the content was contained in a webinar series provided by the client, so there wasn't more than 2-3 hours of research involved in the whole project. It took me about 120 hours, which nets out to $50/hour--substantially lower than my hourly rate on websites, blogs, etc., but worth it to me because I much prefer immersing in a larger project.
Had research/interviewing been required, my rate for the same book would have been in the $11,000 range. Though I haven't won a project at that rate on Upwork, I have been seriously interviewed a couple of times with no request to reduce my rates. On one of those jobs, the person who was hired had bid only slightly lower, and I don't believe price was the deciding factor.
I've written several 10,000ish word e-books for about $1,000 each, though in areas in which I have a great deal of experience and so little research is required.
A few thoughts that may or may not be of use to you:
Upwork (and other freelancing sites) have a large contingent of what I think of as Amazon publishers. They make their money by churning out relatively low-quality content, but taking advantage of the exponential effect of having numerous offerings. They make their money in the aggregate, earning only (in most cases) hundreds of dollars per book, so they have no budget to hire truly qualified writers--and, for their purposes, they don't need us.
I think it's unlikely that you will ever see the type of increased rates for ghostwriting that you reference in your post on Upwork, because most serious clients who come to the site looking for someone to write a book are not in the publishing industry and have little sense of "going rates" and how to discern among writers. In addition, many of them are writing books that are intended to enhance and promote their businesses, not to make money. So, unlike a publisher, they are not shelling out $20,000 in hopes of bringing in much more. The expense, for them, is more akin to buying advertising.
I also think you may be doing yourself harm by speaking in terms of per-word rates. To people outside the writing and publishing industry, that sounds funny and a little outrageous. Surely you've had friends make jokes to you about making thirty cents for writing down "and"? Same perspective, but it's not so entertaining to the guy who feels like he's paying three bucks for a ten-word sentence.
I always bid on a per-project rate. I know other writers here who do well working only on hourly contracts. For the most part, I don't believe that the client base here speaks per-word. There are exceptions, of course, but most Upwork writing clients seem to be from outside the industry.
It might also be the case that Upwork algorithms randomly recommend you to some clients without taking into account your rate, etc. 
