Is important to point the difference between paid advertising, organic advertising and self promotion.
some examples of organic advertising in case you are curious:
1-a scene of your story becomes a meme.
2-word of mouth, people talk to each other about the awesomeness of your story.
3-Your story appeared in the news for some reason.
Self promotion doesn`t neccesarily require money. For example, some writers here do some cross promotion to each other.
Also, in the forums there are topics to self promote. If you are creative there you can atract people`s attention.
You don`t need money to promote yourself, but for advertising and self promotion to be effective, you need to be creative about it.