As a self-publisher, you can spend anywhere from nothing to thousands of dollars producing your books. It depends on your skillset and how much of the work you want to do yourself. But if you're going to spend money, where should you spend it?
Editing
It's difficult to objectively edit your own work, if not impossible. If you have the right friends or a writing group, they may well have the skills to work on this with you, usually on a reciprocal basis. This can be invaluable. Use them to pick apart your manuscript and get it into the best shape you can collectively.
You may want to employ a professional editor, especially if you don't have strong connections. A good editor is worth their weight in gold (I recommend DocBrownTV Editing Services). You have two options here: copy-editors or content-editors.
The content editor will read your story, suggest changes to help improve the flow of your story and pick out any plot holes. You don't have to use their suggestions, but it should give you something to think about.
A copy-editor, also known as a line-editor, will go through your manuscript and find grammatical errors, spelling mistakes, and typos. You may think you've personally spotted everything, but invariably someone will find something once you've published. Copy-editors reduce the number of instances where this will happen.
Cover Design
Absolutely yes, pay someone to do this if you don't have the background to do it yourself. A good cover is the difference between someone picking up your book (or clicking on it) and browsing straight on past it. It doesn't matter how brilliant your writing is if nobody looks at it.
Determine your budget before you start looking for someone. You can spend anywhere between $5 and many hundreds. Of course you pay for what you get: a cover that costs hundreds will be much better than the $5, but the $5 one is going to be much better than what you can throw together with clip art.
Interior Design and Formatting
For the interior of your book, it depends on how you plan you publish your work. It is entirely possible to format your book into an ebook yourself, albeit a bit fiddly. Most epublishers, like Amazon, have instructions on how to format your Word document and they will convert it for you for free. Smaller epublishers may not offer this service, in which case you'll have to convert your file to an ePub or MOBI file, but you can do this yourself. If you don't want to, there are lots of services out there who will do it for you.
If you are looking into print-on-demand services, it is more complicated, but you can do this yourself in Word, adjusting the settings. But it doesn't look as great, so if you're able to afford it, this is a good place to spend money. A Word document will be adequate if you can't afford it, but chances are it will come out looking more homemade than professional. Professional layout services don't need to cost too much and are worth the investment.
Marketing and Publicity
You can do this on your own: Facebook, Twitter, blogs, local libraries and bookshops are all great places to start off. It can be a slow start, but if you don't have the budget for marketing, it's not the end of the world: you just have to be very proactive. Give chapters away for free to pique interest, offer signed copies, run contests.
If it doesn't interest you, then look into hiring someone to run a publicity campaign. Take out ads on social media. You can throw a lot of money at this, but unless you know what you're doing and targeting the right people, then you could simply be wasting it.
So should you spend your money?
Getting your book published requires investment. The great thing about self-publishing is that you are able to determine how much of a monetary investment you want and are able to make. It's easy to just say "I've written the book, now someone else do the rest of the work" and there is nothing at all wrong with that if you can and want to spend the money. But it's equally possible to say "I've written the book, I have no money, I will do it all myself."
Every step of the process is a valid investment if you want to make it. Weigh up the financial cost against the time and effort and skills you have available to determine whether or not you want to pay someone to do it for you and how much you want to pay them. There is nothing wrong with outsourcing your work and nothing wrong with doing it yourself if you're able.
What you need to be careful of is someone who comes along and says "I'll do X for you for X% of your profits." As a self-publisher, your profits are your own (minus what Amazon et al. take from you). Do not give them away. Pay flat fees or hourly/day rates – reputable services all charge this way. We are here to help you create your book and while yes, we want to be paid for our contribution, no one expects your profits.