I think a lot of those fees that shelters are charging are attempts to cover the cost of spaying/neutering. I am most amused that a shelter charges $x and then hands you a coupon for precisely $x good at your choice of any vet on their list towards the cost of the operation.
SPCA a couple of counties over used to (have no idea if they still do) do the surgery in-house and at the most they would hand you a pamphlet listing the daily costs of operation (food, electricity, heat, and yes vet services broken out) and envelope so you could send them a donation - if you wanted to.
stay safe.