I have 2
One who has
About £30 food, £25 insurance £15 vet plan, £20 other stuff eg wear and tear on tiys beds, odd hiring of a field.
Until fairly recently he had £30 a month classes, or the odd £70 training session. he will need a dental every few years (£300 ish) because he's got some teeth stuff
Our other dog! Oh boy.
She has medical issues. She is uninsurable. She is on specialist food (about £80 ish a month), medication (150 a month), supplements (60 ish a month), combined with regular ish vet needs as needs bloods monitoring(probs averages at about £79 a month). She also is a doodle that requires 2-3 monthly trips to a groomer at minimum so that's £20 ! Month min.
She also needs regular dog care, because she cant be left as long as a normal dog can.
Other than unexpected medical costs, the costs that surprised me about dogs is the dog care cost/ fees
In lots of hotels you can pay the equivalent of a human stay for your dogs to join. Centerparks was about a £150 extra dog cost for a 3 night stay, and often your choice of accommodation is limited pushing you into more expensive hotels, air bnbs, cottages. Its more than doubled the cost of some hotel stays for us (eg a travel lodge type thing recently cost £25 for us, £40 for the dogs! Or a haven 9.99 deal thing cost £80 for the dogs alone! ), or we've had to pay loads more for somewhere dog friendly.
Or you are paying for kennels/ dog care
Want to go on a full day trip eg to the zoo, theme park, to the beach etc? A whole day family gathering, wedding? You'll need pop in visits (often more expensive than kennels, but kennels often dont give you a full day care eg 1pm drop off, before 12am pick up or your pay for two nights)
Even prior to the medical stuff with our dogs being good at being left for 6-8 hours, we did find a surprising number of days out go over this, especially if you dont have a friendly neighbour to pop in, or if you dont want to be that person bringing the dog to friends houses. Say you are going to match 1.5 hours a way, 90 min match and half hour break, you'll be over 5 hours without stopping to eat, leaving spare travel time etc, most adult dogs can do that (not all) but puppies might struggle.
This is perhaps more of a thing for us because we are 2 hours from a lot of relatives, so can lose 4 hours in just a days travel. Luckily people are fairly tolerant of us bringing them to family meals etc but we dont want to take the piss