This is one of the most favorite questions put to me. Good reasons - money needed and risk/opportunity to make good money.
Many of the self-publishing/marketing providers you could go to offer differing kinds of services...and that affects the cost to you. Then there are a variety of variables - such as the word count of your manuscript, trim size of the book, the paper stock used, photos to be included, kind of editor needed, having a website, posting on facebook, and more marketing possibilities .... All of these variables can be understood as you have professional help. Often people interested in self-publishing do not get the full financial picture on cost and profit opportunity...in addition to the control you have over the publishing and marketing of your book...and wind up getting surprises because not enough due diligence has been done. You can avoid this by taking that bit of extra time to get objective help and not be taken in by incomplete information.
Anyone thinking of a non-fiction manuscript to get published should definitely investigatge getting good and complete information about self-publishing/marketing before deciding on next steps.