Hello,
I would ask your vet about the following;
Giving sq fluids at home daily, adding a short course of a steroid and the latest opinion on prazosin’s efficacy. (I don’t use it).
Also I don’t use vinegar.
These are a matter of experience and preference. Therefore please ask your vet or find a feline only practitioner to help.
I’d guess the low cost clinics don’t do pre-op bloodwork? Furthermore, costs of everything have gone WAY up, and shelters/rescues are funded with donations and fundraising efforts while veterinarians have businesses to run and employees to pay.
I find $400 to be quite fair. We paid around $700 for our Doberman’s spay about 6 years ago.