writing has to be among the worst ways to make money, sorry to say you need top .5% talent and work ethic to maybe earn a lower-middle class salary A labor of love, as it's said
Writing is a solid way to make money in specific genres, however, especially if one is self-published. A business-saavy writer can make a good amount of money writing romance (which makes more money than most other genres combined) or self-help.
My wife is in marketing for her day job and has been using that knowledge to help target and generate interest in her books, and it seems to be paying off, as her preorders are eclipsing quite a few established authors in the groups she's in, and this is her first book.
When she had half the preorders she does now, a friend was saying she could probably expect around $2k in sales in her first month, judging by the preorder numbers, so by the time it releases she might be seeing 2-3x or even more than that.