Not all rescues are the same. Some rescues are operated like a business and take in donations, both monetary and non-monetary (food, bedding, supplies), some are operated by individuals right out of their houses. These are ones that typically don't get a lot of money or donations and may need more monetary help with a dog's medical expenses. As I mentioned above Cherry needs a dental (which can cost $200 +) but we don't have to pay for it and we're paying $125 to adopt her. A smaller rescue may not be able to absorb this cost and need you to pay it. You might see this more with a breed specific rescue.
If anyone is interested in adopting a rescue dog do some research. You'll find there's quite a few out there and their adoption prices vary. Also, you might get a discount on a second adoption from a rescue. We're getting a $50 discount from our rescue because we adopted Myrtle from them. And adoption fees may vary by age (younger dogs typically cost more to adopt), or if there's something wrong with the dog. Before we put our paperwork in on Cherry we were looking at a chihuahua/beagle mix that had a front leg deformity that had an adoption fee of $25. Someone put in an adoption app ahead of us and took (good for him).