When you do any sort of promotion, you need to judge its cost vs. benefit.
Of course, it's rare to actually recoup your time/money investment in any kind of promotion, let alone profit from it. But authors know they have to build brands, and you have to spend money to make money.
That doesn't mean you have to be stupid about it.
Before you do a single thing to promote your book, ask yourself this question:
What is it you want to accomplish?
The more specific your answer, the better job you have of attaining your goal. "Selling 30 books" is provable. "Building name-recognition" is not.
When you've decided on your goal, the next question you need to ask is:
What works on you?
So many authors pursue self-promotional venues without actually thinking about them. There are reasons for this:
1. They know they have to be doing something to promote their book, and it's better to do something than nothing.
2. Everyone else is doing it, so it has to have some merit.
3. They've given zero thought to expectations and return on investment.
The problem with people in general, and the publishing industry in particular, is that very little time is spent analyzing why they do the things they do.
Human nature tends to lean toward self-preservation. This means that people spend more time trying not to look stupid, and justifying their actions, than they do actually thinking about options.
Why do authors go on tours when it's obviously cost ineffective? Why do publishers buy huge ads that could never pay for themselves? Why do authors continue to hire publicists when their fee is never justified by books sold above and beyond what would have been sold anyway?
Because that's the way things have always been done, and humans would rather make excuses for wrong behavior than figure out better mousetraps. No one wants to make decisions, because that requires culpability. So we allow decisions to be made for us by following the same ineffectual paths, and then spend our energy rationalizing their failures.
When I do any sort of promotion, I follow this formula:
1. Decide what you want to accomplish.
2. Decide how much you'll pay to accomplish that, and be able to defend your decision.
3. Figure out a way to judge the effectiveness of your effort.
If that's too complicated, save your time and money and don't promote.