If you are looking for low cost you can try
www.friendsofanimals.org they offer Spay/Neuter Certificates that will give you a considerable savings at a participating vet office. My girlfriend just used the coupon and said the spay was $10 total!
If you are looking for a low cost spay/neuter clinic, you can contact your local animal shelter or SPCA and ask them for info. They usually have their animals spay/neutered at the low costs clinics so the should be able to refer you.
Good luck!!