I have 2 small dogs both are puppies though so the prices might be different when there adults.
Food - dry food one bag lasts a month which is £7
wet food about £12 a month
treats prob £5 a month
Flea treatments & wormers - works out at £5 a month
Insurance - mine are still under their free insurance but the quotes i have are £8 a month each. But it goes up when there older to about £20/30 a month each!!
Groomers - Ive learnt how to do it myself but before I was taking my fluffy dog to the groomers once a month and it was £12 each time (it goes up when there fully grown) so thats worth taking into consideration.
Vaccines - initial ones are £40 each then £20 a year after that
neutering - starts at £60 for boys and £100 for girls, goes higher the heavier they are but is cheaper if your on benefits.
Initial outlay - bloody fourtne in Pets at Home!!! I cant even count how much, crates, beds, leads, collers, id tags, grooming kit, bowls, toys the list goes on though in sure you could do it a lot cheaper
So prob about £30 a month each, but it can prob be done a bit cheaper. If you fed them only dry food etc.