Consider this: Even writers as great as Edgar Allan Poe had to fund their own poetry publications because the publishers would not touch poetry without money up front. Short stories and novels, sure, but not poetry. Nobody has ever gotten rich with poetry books except the offspring of Kahlil Gibran. His books sold fairly well, then made a huge resurgence in the 1960's. He did not live to see the money. Publishing poetry is art for art's sake. It is not reasonable commerce.
Some self-published books have quietly developed their own following and have sold very well. There are a few millionaires who have done so, mostly with romance novels. 50 Shades of Gray is one example. It originally began as a self-published book. It did well enough that a big time publisher bought the rights and made it a household name. It is not common but it is possible, more so in the romance novel arena.
Short stories are similar to poetry but not as bad. Novels are the best. Books sell in this order, as far as I understand it:
Teen Coming of Age Novels
Teen Romance Novels
Teen Paranormal Novels (Like Harry Potter)
Any Young Adult Novel
Making $50 to $100 per month on a book would be a very successful self-published book. Eventually, it should snowball into something more. There is no telling how long that will take.
There is money in such books, but do not expect to use one to put your kids through college and don't go quitting your day job.
Poetry Books will sell, but the sales will be slim and spotty.