One thing I've learned is if you're an author and you just add expenses as you go, you end up spending a lot more than you planned. Eight years into this gig, I'm a lot better at planning ahead. If I'm not releasing a new title, if I'm just promoting the books I already have, this is what my budget currently looks like. As you can see, it still adds up, but it actually only ends up being $22.75 per month. Per the Vegas Rule, this is spending what I can afford to lose: about the price of a pizza. Keeping costs low means that even if I don't sell any books at all, I'm OK. If I do sell some, it's a bonus.
Every author will have a different budget. As you can see above, there's a cost for a year's worth of my website, a cost to keep the domain name (www.amieryan.com), about 80 for advertising (although I do try to get freebies online whenever I can--the image above was created, totally free, with a site called Canva), the cost to keep Starfish on the TaleFlick site for another year in the hopes it will get optioned again, and the cost of entering one writing contest, just for fun.