If rescues didn't charge the amounts they do, a lot of them would go bankrupt. We charge $350 for a dog, and that doesn't even cover a lot of the dogs we get in--ones with mange, parvo, broken bones, or aggression issues that need special training before we consider them adoptable.
Yup, sometimes we get the magical healthy puppy who we don't have to spend a cent on who's adopted in a week--but for every one of those, we have 5 dogs with mega vet bills to make up for it.
As it is, we don't cover our costs with adoption fees. We fundraise like crazy, have sponsors for our events, and still rely on our volunteers to give generously. If we lowered costs to a "reasonable" $100, we would no longer be able to rescue.