I suggest contacting the Humane Society or some rescue groups in your area and ask them for suggestions. I'm up near Sudbury and I'm working with a rescue to help spay/neuter feral and stray cats in the area. I believe the vet that this rescue uses charges $100 for a spay (versus about $300 w/ my usual vet) and I'm pretty sure that's including vaccinations. You should even be able to find something cheaper than that.
And I bet she'll be fine. Most spays go well without any complications. I've had a few cats done as well as dogs and they have all been fine. As for heat cycles, I really don't think this is hurting your cat. I've heard of some animals getting some cramping similar to what we get, but I doubt that most animals experience this. I believe the "yowling" is more of a mating call than a cry of pain. She's just looking for a tomcat