Does that include everything:
Pre-anesthetic blood work
I also do a buccal mucosal bleeding time test with first surgery dogs and ask that any MDR1 drugs are avoided. In addition, big pain in the butt here requests no NSAIDs for pain management.
Is there someone there while the dog is being held overnight? I know my vet office doesn't have someone there the whole night (but usually with emergencies there is coverage for a good part) and generally ask for overnights for most spays, but they will allow owners to take the pet home if they believe the owner will follow instructions.
My vet charges for some things by weight, so the heavier the dog, the more anesthesia, etc, they use.
I honestly do not know about the processes and costs. My vets have spayed for rescue for younger and older dogs and the costs have varied greatly by weight. Neuters on young males are definitely easier surgeries in terms of their recovery, but a young female and a spay is usually feeling well too soon - they want to do way more than they should for their stitches! Sorry for the roundabout non-answer.