Yeah, I have to agree. I know alot of people on the various cat boards have said their cats also stay overnight after a spay, but once they are awake from the anesthetic there's no real reason a cat has to stick around, and they are usually awake within a few hours. I have also always felt that a cat is more comfortable recovering at home. Since I used to be involved in rescues and TNRs, as well as having had lots of my own cats, I have probably been involved in about 20 spays, and none of them stayed overnight or had any kind of complication. So if money is a factor, tell your vet you'll pick her up that same day. EVEN SO, the boarding fee is generally $10-20, so even staying overnight does not account for it. My costs for inintial exam, spay, and shots generally is around $120, so even if they stayed overnight, it still would not top $150.
I think maybe another vet is in order too, especially since the cost is so high your cats are going without medical care.