They're transported in from wherever they've been, are fixed, evaluated, gotten back to health, and (looks like) individually fostered. I'd challenge anyone to do that for a single dog in Maryland for under $400. I ship dogs relatively frequently and it's up around $250 now just to ship.
Rescues, especially breed specialist rescues, have the right to be extremely choosy about where they put their dogs. Their responsibility is to the dogs, not to applicants; most of their dogs have been brought in from great distances and have more specific needs or special circumstances than the stray or owner surrenders at pounds or shelters.
As a breeder with a very extensive screening process, in a community of breeders who do the same, I can tell you that we're not looking for the rich folk or the ones who impress us with a home or car. We're looking for a commitment to the dog's welfare that is equal to our own. If that seems overly picky that's fine. I've given away puppies and I've taken payment plans on puppies and I've gotten checks from people who drive brand-new Porsches. The Porsche-drivers don't "win." They happen to be good applicants, so they get a puppy. The person who says they can only give me a few hundred a month who happens to be a good applicant gets a puppy. Sometimes the reason they can only give me a few hundred a month is because they spent money to put up a safe fence - it's happened, and I'm thrilled with a committed owner like that.