It depends on how much work you're willing to put into the dog. Some small dogs have a high maintenance grooming requirement and must be clipped every 6 weeks (around $50/groom), and they need to be brushed daily or nearly every day. I just adopted a westie/poodle mix, and we love her dearly. However, westies have notorious allergies, and Maddie (12.4 lb.) is no exception. Our miniature poodle (12.8 pounds) is also a rescue and he's cuddle bug! However, his hair grows like a weed, and he needs clipping every four weeks. Fortunately, I learned to do it myself, saving a lot of money.
The initial clippers, blades, etc. are expensive, but they pay for themselves in only a few grooms.
Some small dogs have short hair, lessening the grooming requirement dramatically. Some small dogs are high energy, so if you want a couch potato, beware.
I recommend getting an adult rescue from a reputable group in your area. They've been vetted, spayed/neutered, and have had their shots. They've also been temperament evaluated, so you'd have a better idea of what you're getting than if you got one from the pound. Just be up front and honest when you talk to them in letting them know what your lifestyle is like, so you get a good match.