I dread to think he much our dog has cost us, but we have definitely gone for more expensive options, so don't take what we pay as given, I'm sure it could be done more cheaply at no detriment to the dog.
Also worth bearing in mind as a general rule of thumb, the bigger the dog, the more expensive things are. We have a big lab (approx 35kg at just over 10 months!) so he gets through more food, the wormer is more expensive as its stronger, the toys need to be bigger etc.
Food and treats: approx £100 per month
Insurance: £40 pm
Wormer/flea: £10 pm
Bed can be got a cheaply as you like, use your imagination to make it comfy. As nice as the sheepskin ones looked (and for over £100 you'd hope they would be) we didnt want to spend that much when we knew it was likely to get chewed and outgrown quickly, so we got a hard plastic one and a cheap duvet, with a fleece/sheet thrown over in winter/summer. Mine also loves his cheap pillows to lie on.
Collar and lead can be got for under £15, but as with anything, you can spend as much as you like.
A brush, toothbrush/paste/dental sticks, old towels and blankets are also pretty essential, but not expensive. Shampoo and a rubber bath mat are advisable for when they inevitably roll in fox poo!
Toys - it depends on the dog. We have to spend more on the 'indestructible' ones as our boy can rip a cuddly toy to pieces in seconds. An antler is a good investment, while they're expensive (arpund £15 for a small one) they will last years compared to to bones.
Our vet is relatively inexpensive - £25 per consultation. However, emergency vets are horrendously expensive - we paid £200 when ours got a bit of plastic stuck in his bottom on a sunday,which took the vet less than a minute to remove. Booster jabs are annual.
I often walk with a dog walker who charges £10 per walk, which I think is the standard rate. Our training was £100 for a 6 week course (1hr x6), one to ones with the trainer were £35 per hour.
Don't under estimate how much work they are, in addition to the expected walking and attention, your washing and hoovering will also go up.
I agree with others that you're best off speaking to the rescue centre about whether there is a dog that would fit your lifestyle. And have a think about whether it might be possible to change your hours or come home at lunch etc to accommodate a dog.
F you can afford it, and can adapt your lifestyle to give a dog the attention and care it needs, they're wonderful, rewarding pets, so I hope you can!