I don't know where you are, but in Connecticut, they have the TEAM (for Tate's Every Animal Matters). It's a van that goes all over the state (there are actually two of them) and spay/neuter cats (not sure about dogs) for low cost.

$66 gets you:

Spay/Neuter
Deworming
Vaccinations
Trims nails
Earmite treatment

You drop your pet off in the morning and pick him/her up at the same time later that day. Cash only. No checks or credit cards.

This is a remarkable thing for all the residents of CT. You'd have to put down at least $40 alone for a vet visit. At least where we are it would be.