I used The Big Fix Clinic or Mississippi Spay & Neuter Clinic which only charged $50 per cat, plus if you want $8 for the rabies shot. I think they have some grant money plus donations from the public that helps with the cost. Both of the mornings were very busy with cats and dogs coming in for their surgery. Dogs cost more, but still cheaper than the vet.
Native Texan, I know what you mean by not being sure if they are pregnant. Since Mama only had one kitten at a time, I never was sure if she was pregnant until it was too late to do something about it. At least when a dog goes into heat you can tell immediately.