There's been a couple of threads on this recently
www.mumsnet.com/Talk/the_doghouse/3148198-Realistic-costs-for-owning-a-dog
www.mumsnet.com/Talk/the_doghouse/3155946-How-much-does-your-dog-cost-a-month
Costs can vary enormously based on the breed, size, quality (e.g. of dog food) you go for, how quickly they destroy toys, whether or not you have willing unpaid dog sitters / go on holiday often, need a dog walker... probably best to do your own costings really based on what's plausible for you and your lifestyle.
PestDog is a recent addition to my budget, but so far it's looking like
Food - Markus Muhle (found out about it via AllAboutDogFood.co.uk; it's an unusually good balance between nutrition and price) which reckons 18p/day £5.40/month or but I suspect he will need a bit more as he's quite energetic.
Training treats - probably works out at up to £2.50 per week, but we do a lot of training and use high quality dried offal
Other chews - doesn't get one every day, but depending on what I give him, it can be 30p-£1 per chew (high quality, natural dried meat things)
Pet insurance - £42/month (small crossbreed, of a type not especially prone to health problems) using PetPlan Lifetime, so it won't really go up and will cover him for life if he gets diabetes or something else long term and expensive.
Wormer - £2/tablet every three months
Flea treatment - haven't actually worked this out as I still have a stash from the last owner
Boarding - I have family in another city who willingly take him, but the return fare (£45 x 2 to drop off and pick up) means it's not worth it for a couple of nights. Per night of home boarding, no other dogs in situ, with his qualified trainer is £30/night (but it doesn't get much cheaper locally even with unqualified randomers doing it as a sideline). Even if you can take your dog on holiday, be aware that some places charge extra for your dog to stay, though it will be cheaper than boarding.
Toys - I've had to build up his supply from scratch in the last 6 months, and frankly I don't want to think about how much it has cost me - I'd estimate £2-300, but many of the toys are still alive so it's not going to be that much going forward.
Training classes - £10pw, though for various non-cost reasons we're switching to one that's £6.50pw. They're group obedience classes.
Coat - equafleece, £25, still going strong so should last for at least a second winter if not a third.
Collar and lead - £10 for the basic. Whizzy rechargeable LED collar for night time walkies was another tenner.
ID tag - £2-something from eBay
Poo bags - £7.99 for 270 biodegradable bags. 3-4 bags per day, so 8-11p per day
Bus fares to get to and from certain parks - variable, depending on whether or not my Oyster has been capped for the day. Factor in petrol if you intend to drive. Do be aware that your local park may be locked at dusk, which is a pain in the winter if you work, as it means you may have to travel to an alternative.
... and then there are all the little things you don't consider such as wear and tear on your walking boots (they've seen more wear in the last 6 months than they have in the 13 years since I bought them for Duke of Edinburgh ), a good human coat for walking, waterproof trousers, extra hot water when your dog is a mud magnet...
No grooming costs as he's a short haired dog; I don't even use dog shampoo, I just put him in a bath of plain warm water when he's muddy and let it soak off
No need for nail clipping as he walks on tarmac a lot and occasionally gives them an extra trim when he scrabbles after a fox or cat
We don't use a professional dog walker
Haven't had to deal with vaccinations yet
Already has a pet passport, but I gather that cost the previous owner £150
That brings me to a ballpark figure of £120 per month + boarding + one off / infrequent costs like coats + bus fares.
I think PestDog might have to get a part time job to pay for it all