I have a few friends who breed and show dogs. The costs of keeping a breeding bitch can be enormous. There's her feed, grooming, vaccinations, health checks, scans, insurance, cost of setting up a whelping room (whelping box, heat lamps, vetbed etc), vet bills in the event of a difficult whelping and someone needs to be at home with the bitch and puppies virtually 24/7, from the last couple of weeks of the pregnancy until the puppies are at least 6 weeks old.
Take my breed, the lakeland terrier. The average size of a litter is 4. The stud fee is the price of one puppy, or pick of the litter, so that leaves 3, assuming they all survive. The costs of all the tests, vet care, special food, registering the pups etc practically wipes out the price of another pup. If the bitch is unlucky enough to need a c-section, you're looking at a minimum of £1,000.
If any of the puppies show "faults", eg less than perfect bite, slightly the wrong sort of coat, top line less than straight, a decent breeder (which, thankfully, lakeland breeders seem to be) wouldn't dream of trying to get full price for them. The chances of getting a litter of entirely perfect puppies are slim.
I haven't looked at the price of a well-bred lakeland pup for ages, but I'd guess they're around £850-900. When you offset the stud fee and the costs associated with whelping, you're left with 2 puppies that you could sell, giving you £1700-1800 net income.
No reputable breeder would breed more than one litter a year from any one bitch, or more than 5 in the lifetime of the bitch. So at the absolute maximum, given that this breed can easily live to 15, even 17, breeding puppies could give you a princely total income of £9,000 over, say, 16 years, so £500-600 a year, and only then if you're lucky and every puppy makes it. Barely a tenner a week (less cost of food, insurance, grooming vet care), for spending hours looking after a dog in the best possible way for its entire life.
And if a puppy buyer later decides that they can't keep the puppy for any reason, at any age, the responsible breeder will take that puppy back and either keep it for the rest of its life or find a good home for it.
At best, breeding a litter, when done properly, subsidises the breeder's hobby.
Some breeds are probably more lucrative. Rottweilers, for instance, are renowned for being great breeders and they have massive litters (I've known a litter of 13). Some are probably a lot less so. Bulldogs are notoriously difficult to breed, the bitches suffer from dystocia and they have to have c-sections in 80-90% of cases. And bulldog breeders often use AI, as they struggle to mate normally.
Of course, this is all very different from the backyard breeder who churns out litter after litter of dubious crossbreeds, with no thought for their temperament or welfare, and sells them through Gumtree for £300 each. They're just scum imo.