Poking around on Dogster, as one does, I found an article about the Toby Project. The Toby Project is a non profit, low cost spay and neuter van that travels New York City.
The spay or neuter (and rabies vaccines, apparently) are free to New York City residents who are on public assistance. They're also free for "pit bulls and non-purebred and cats". For purebreds and "designer breeds", the spay or neuter is $100 and the rabies vaccine is $10.
The November calendar for the Spay/Neuter van can be found here, and you can call 212-799-1120 for information.
At prices like that, and with the information readily available, cost is no excuse. It's cheaper to go spay or neuter your dog or cat, no matter how awesome that individual may be, than to responsibly breed a litter. It may or may not be healthier for your dog or cat; there are opinions on both sides of this. A female dog who has been spayed cannot get uterine or ovarian cancer; a female dog who was spayed before her first heat has a reduced chance of mammary cancer. A male dog who has been neutered will not roam the neighborhood to find the female in heat that he can smell, will not get testicular cancer, and may or may not be more mellow in general (though dog behavior is frequently a training issue rather than whether the animal is intact).
Spay or neuter your pet. Elka says it's the responsible thing to do.