Many areas have Humane Society or Health Department grants to reduce the population of feral/free-roaming cats, and for "homed" cats to reduce over-all populations.
I know people who routinely trap free-roaming cats in their neighborhood and take them to these clinics. One of my co-workers has taken dozens of cats. The surgeries are done by vets who volunteer, and the grants defray their costs. My co-worker feeds these cats and watches them carefully, and there has never been a problem afterwards.
I agree, it would be generous to donate, given the amount of money you will be saving.