Do some research on dog food. I would definitely not just get the cheapest stuff. Usually it has less nutritional value so your dog eats more (and poops more), it's less healthy, etc. But, that doesn't mean you'll have to break the bank either. Our dog has food allergies -- you know, I used to make fun of people who's pets had food allergies, it just seemed so manufactured. He can't eat chicken or he scratches himself bloody. So we buy a special fru-fru brand (Wellness) that's grain free and poultry free, and we still only spend $30/mo on his food (20 lb dog). We usually buy it from Amazon. Buying in bulk is super tempting, but be careful. We did this once and then ants found the bag... that was such a mess.
Toys - to start with we got a a couple toys, but nothing expensive. A squeaker toy, a chewing toy, a tug toy, and a fetch toy. $5 is the max I spend on toys. Squeakers die, but you can get replacements instead of buying new toys. We shopped the clearance bins for toys. And we got a toy and leash from ROSS dress for less. And to be honest, our dog LOVES cardboard. He hears when the last of the TP gets ripped off the roll and is SO excited to play with his TP roll (until he starts shredding it and then it gets recycled). Egg cartons, cereal boxes, kleenex boxes, toothpaste boxes... he loves them all. Our family also likes buying toys for our dog, so that's something to keep in mind. If you have gifty people in your life this could be a good outlet for them.
Leash/collar - we just have a nylon leash and a nylon collar. We still have the leash from when he was a puppy and it still works. We also have another leash that just showed up. I think my husband found it in a free box. It is handy having two leashes because often one is MIA for weeks at a time. I think our sock gnomes are branching out.
Flea meds - we found some on Amazon for super cheap... but sometimes some of the tubes were empty. So that was weird. Right now we're trying something "natural" for flea control: diatomaceous earth. It's working so far. We go to the dog park and it's still keeping them under control, but we also hose him off after we get home. If you do decide to go this route: DO NOT overdo it in your house. Use the duster and wear a dust mask. If you have allergies or asthma, it might be hard on you. I think it's been 3 months since we started and we notice fewer fleas than when he was on chemicals control.
Puppy Romps - This was SO fun. Our humane society has puppy romps once or twice a week. And once your guy has all his shots, you bring him down to learn how to socialize with other dogs. It's ADORABLE. There are puppies everywhere! It's by donation, so we spent $5 on it each time, but it was so much fun. We loved it, he loved it, and I think it really did help him behave appropriately when meeting dogs for the first time. And you can talk with the staff and they can give you pointers and advice.
Training - we did training through Petsmart. I think it was $100 for like 2 months. They use a combination of clicker and treat training. I think it was worthwhile. I've never had a dog, so I didn't really know how to train one. There's definitely some things (sit, stay, come) that are important safety things for your dog to know.
Dog door - If it is at all possible, I would get a dog door! House training was SO easy once we got that thing installed. We were using a bell (I hung a jingle bell from a ribbon around the back door knob). The dog would ring the bell and we'd let him out to do his business. It worked well, until the dog realized that if he just wanted to lay around outside, all he had to do was ring the bell. I LOVE our dog door. No accidents, and it's super convenient. And we can replace that door for $100, assuming we buy it brand new.
Carpet cleaner - we bought a small carpet shampooer, but a bottle of Resolve probably works just as well. Often we would use the resolve instead of the shampooer because it's really kind of gross. It sucks up the mess and then either you empty it immediately, every time, or the mess just hangs around in the shampooer...
Poop bags - We re-use vegetable bags as poop bags. But we also get poop-specific bags from Amazon. We run out of the re-purposed bags otherwise. And it's handy to be able to grab a whole roll when you go to the dog park or on a walk.
Gentle Leader - We never mastered lose leash walking. Our dog is a terrier and they are so focused and stubborn... He would pull so hard he'd be choking himself. We got a gentle leader and it's amazing. It looks kind of like a muzzle (there's a strap over the snout), but they can eat and drink and pant in it just fine. But when they pull, the leash is attached to their head, so it's super uncomfortable. He still pulls when the leash is on his collar, but as soon as the gentle leader goes on, it's like night and day. He hates it of course. He'll paw at it to try and get it off. Some dogs hate them so much they lay down and refuse to move, but our guy is too hyper to try that approach. We also got a harness from a neighbor and that works too. But the gentle leader is so small, it fits in a pocket, so it's really convenient. And I think it's faster to put on (once you figure out how).
Pay attention to the dogs that walk by your house. If you can, talk with their owners and and mention that you're going to be getting a dog. They may have old puppy stuff (tiny collars) that they don't need anymore. Or toys they bought for their dog that their dog ignores. Or treats that their dog refuses to eat (or can't due to allergies).
Good luck! Dogs definitely add some expense to your budget, but every time I come home he is just SO happy to see me. I love watching and playing with him. If I'm sad, just looking over at him makes me smile. And when he growls in his sleep... it is the cutest thing I've ever heard in my life. It's adorable. The joy I get from my dog is well worth it.