This is a timely question for me, as this month we just adopted our second shelter dog (we had the first for 14 years before he passed away).
We adopted a 40 lb, 10 month old, black lab mix. She is very sweet to us and strangers out of the house, but we discovered after bringing her home that she has some fear-based territorial aggression (ie., she barked nonstop, snarled, growled, etc at people entering our house or yard, even after trying to quiet her, letting visitors into the house for awhile, etc). She shows none of this to strangers outside of our property and no forms of aggression towards us, but we don't want to have a dog we can't have around people in our home. We brought in a behavioral specialist right away, have been working with her, the dog has made great strides already (using all positive counter conditioning and desensitization if anyone's curious), and it seems like she'll be able to get over this issue. My point is that one should plan on unexpected issues with any new dog (shelter or not)and be prepared to spend the money in the beginning in order to deal with issues upfront to ensure everyone in the family a great next 10-15 yrs.
So far, we've spent on her:
adoption fee $150
initial vet visit $100
crate, haltie training head halter, collar, leashes, brush, toys (esp chew toys to keep her from chewing up our house), dog tag, etc $250
house visit of animal behaviorist $100
dog training class (very needed for socialization) $150
So, $750 upfront. And while it might not seem mustachian, if one if going to have a dog , IME, spending similar upfront on proper training and training equipment will save in the long term. We are also spending about $50 a month on her food, treats, and maintenance items (tractor supply 50 lb bag, cheap treats, small pieces of cheese for training, flea/tick meds, etc). We will be boarding her over thanksgiving and xmas (total of about $180) because we don't feel that she's ready for large crowds and people coming and going yet. But, hopefully, after that we can take her to visit family, etc and just board her once a yr or so.
So, after upfront costs, I'm thinking she'll cost about $600 a year for food/maintenance, another $100 or so for boarding, and $200ish a year for vet bills (expected average over her life). So, about $900 a year, or $75 a mo after the startup. To me, a dog is totally worth $750 upfront and $75 a mo, but one getting a dog should mentally plan for spending around this and avoid the temptation to go cheap in ways that could wind up resulting in longer term expenses, unhappiness, or even devastation (eg., the dog bites, gets hit by a car, seriously hurts another dog, completely destroys a room when left alone, etc) that could be avoided through spending adequately on proper training, equipment, and boarding/care when needed.