My wife and I had a 7 pound Yorkie from our local SPCA that died pretty recently. As a little dog, especially a Yorkshire terrier, he definitely had his medical bills. But outside of those expenses, our month-to-month costs were almost negligible. A $25 bag of food would last 8-10 weeks. Any medication we got him we broke in half or sometimes even quarters, because they literally didn’t make doses small enough for him, which meant any course of any medication was pretty cheap.
I mostly bring this up because a lot of people assume little dogs are incredibly expensive because of medical issues. They are not wrong per se, and breed matters a lot here, but there are plenty of small and healthy mutts that need good homes and your monthly budget for them is going to be much lower than what you need for some of larger dogs. Plenty of breeds are hypoallergenic, only need short walks, and are overall just not very demanding. Something to consider!