
Rearing puppies properly, stud fees, vet expenses, health checks etc are costly. The last average size litter i bred, with a C section cost me £1200, and that is not taking into account washing bedding electricity etc. That works out at £200 per puppy.
I would be highly suspicious of puppies being offered for les than £300, as a lot of shortcuts would have had to be taken, or the breeders are wealthy.