Not saying you should sacrifice quality healthcare for money, but that sounds way excessive. In Lafayette Louisiana the most expensive guy in town is $200.00, and there are at least 4 that do spays for $75.00. My girl Stella has no scar at all, and was done for $75.00. More money does not always mean better care. I have a friend with a lab who has been battling a skin condition on his Lab for years. He has spent thousands of dollars and has had no success at getting his dogs ailment healed. I passed on some advice from my "cheap" vet and within a month the affected area on his dog is almost completelt healed. All I told him was what my vet recommended about incorporating canned food into his diet. My Beau had extremly dry skin, and was scratching off patches of hair, and my vet told me to give him a more oils in his diet with canned food, and it cured him.
As far as staying overnight I woud make him give me specific reasons why, because anthing could happen. I also wouldn't want to run the risk of my babies being sorrounded by other pups who were sick. I would insist your vet show me his faclities, and make sure your dog is not in the company of sick animals and that someone is present. Look for things like how clean are the holding cages or areas.
What I would really do unless you have a strong reason for using this vet, is shop around. It sounds like you are getting the price for two spays minimum.