My mom started a dog rescue when she retired and it's a lot of work tbh. You have to have a country vet, in the country, who doesn't charge a fuckload of money because you get dogs with heart worms, parvo, etc. every disease in the book that spreads to any other dogs you have. If you have to quarantine after a nasty disease you basically have to take care whatever animals you have at the time for a few months until you're sure none of the animals are sick before moving them.
That's on the back end. On the front end you have to deal with all the fucking people you're placing animals with. That's hundreds to thousands of applications (over a year) from people you have to track down, talk to, verify, and meet with, let alone all the flaky people in the world who cause you inconvenience.
THEN there are returned animals. Some fucking dogs are just not place-able due to behavior or long term health problems, or even too much energy................................................................................................................................................................. and those people come crying to you......I had a lady I drove to meet to place a dog and it "panted" too loudly. Meanwhile you turned down other applicants you now have to contact to move the dog to get any money after eating whatever trip you made for nothing to the first person.
So yeah, you can get $350-$550 for a "free" animal after a lot of work and expense. Some animals (poodle crosses lol) are simple to place and take minimal effort, but it's still a lot of work and shit to put up with.
You also have to have a network to move animals and treat animals, when you find a cheap vet they can only do (spay, neuter, treat) so many of your animals in a week.... Also you have to be a "wheeler dealer" type of personality. My mom is mystifying when it comes to selling shit.