If I've finished my book, had it professionally edited, and am ready to self-publish my book, how much would it cost for a paperback, 300 page novel? I've already hired an editor, and I'd like paper copies. The length of the manuscript is 78,649 words.
Since it appears that you are interested in self-publishing in print format, the answer to your question will depend on where you go to have it printed. There are companies such as CreateSpace and Nook Press which will allow you to create your print book and buy copies yourself or just make them available for Print on Demand (POD). These are the two easiest formats I have found, so I am going to answer using them as examples.
With CreateSpace, you can use the "Buying Copies" tab on their Book Product page to see how much it would cost to produce each book. Using an estimated page count of 300 pages, with black and white print, and a book size of 6x9, your cost would be about $4.45 per book. You could also choose to make your book available through POD at no cost to yourself at all.
With Nook Press, you can use their Quick Quote tool to find out the cost for producing each book. Using an estimated page count of 300 pages, with black and white print, a paperback cover, and cream colored paper, with a book size of 6x9, your cost to produce each book will be about $6. I haven't used this service, but I believe you can make your book available through POD with them and it won't cost you anything at all, just as with CreateSpace.
The advantage of using each company's POD service is that you don't have to spend any money yourself, plus they provide a sales platform for your book. Both companies also provide an easy means of making your book available in e-book format, which means you have the potential to reach an even larger audience.
However, if you are only looking to obtain copies for yourself and a small group of family or friends or such, then either of these services would be worth looking into.