There are many free/low cost spay neuter clinics. Call your local humane society or shelter and ask about it. One near me charges 25 for males and 45 for females so i'm taking mommas and kittens there when they are ready.
Sadosi, it wasn't meant to make anyone feel bad for having intact cats. I have one that's expecting as well, but she'll be fixed when her kittens are gone.
I would do as Lindsay suggested and check out what is available in your area for reduced cost spaying.