As I believe has been mentioned in previous editions, one of the greatest advantages of self-publishing is the amount of control that it affords you as the author. Amongst other things, this allows you to set your own prices. Knowing how much money you have spent on putting your book out there, you can then work on pricing it effectively, to maximise your profits.
Another such advantage associated with self-publication is the higher percentage of returns from royalties that authors receive. In some cases, there are no royalties at all, meaning that you get to keep 100% of your sales profit.
In cases such as these, there are still expenses to be paid - but the initial outlay of funds usually makes more economic sense than an ongoing financial commitment.
And speaking of expenses, one of the best ways to ensure that you are making money from your manuscript is to consider and choose your investments wisely.
Professional assistance with the marketing for your book would be one example of a worthy investment. Again, this would probably involve a one-off, possibly upfront, payment, but it is almost always worth it in the long run, for the returns it will yield you.