Spay and nuter your pets, and it won't be a fraction of the problem that it is. I'm actually involved in a program when I catch feral cats and get them fixed.
What's the average cost of fixing a cat, relatively speaking? Think you could guesstimate one?
I go to a wellness clinic. Since they're not a vet, and only help keep healthy animals healthy (vaccines, tooth cleaning, neutering, flea drops, etc...) it keeps the costs down. Spaying/neutering there costs $40. I know it can be as high as $100 at a regular vet, but it's usually cheaper than that.
I'd really recommend that anyone with pets find a wellness clinic in addition to their vet. Besides cost, it helps keeps your critter from being exposed to sick animals.