The monthly upkeep cost of a dog is irrelevant if you can't afford to pay for medical care and other unforeseen problems. Would you be able to afford a major surgery or other emergency treatment? Basic monthly costs depend very much on how and what you choose to feed, how much equipment and supplies you buy, what level of routine veterinary care you pay for (vaccines, minor issues), and so on.
Starting out on a "tight budget" isn't promising. Where are you going to get your puppy? Is it a reputable breeder charging $1200 or more, or is a backyard breeder selling cheap puppies that may be riddled with expensive major health issues later in life?
The Cost of Owning a Dog