There is no monthly fee, the only fees ever charged are when you make a sale and that's usually already rolled into the print cost of the book. Or if you're doing e-books, the fee is only taken when you make a sale as well.
I market on my socials. I also did a little giveaway promo when I launched my first novel. I offered a free book kit for people who filled out a google form when they preordered a physical copy.
I've acutally talked about this as well as the rest of my self-publishing journey in this past thread!
Nope, never done a kickstarter. But I have the benefit of having a healthy savings account. If you do need funds that you don't readily have, then I do recommend a kickstarter.