Purebreds form a reputable breeder will cost over $1000 easy. So a purebred from a rescue for $300 could be considered a bargain.

Really if you break it down $300 can be justified. Like you said the dog will be spayed or neutered and have his shots. Some of the dogs that they get may be sick and need vet care- possibly surgeries and medications. Some of the dogs need a great deal of grooming. They also have to feed the dogs. They have bills to pay too- rent, power, phone, etc.