There are spay and neuter clinics in my area that charge only $15 to spay a small female dog! And with excellent equipment and trained vets like any vet office. =)

I'm glad you are being responsible for your dog by spaying her, having rescued a gorgeous little Chihuahua girl from near death at a county shelter a few years ago (she is now loved by my sister and her family), I have experience with seeing an abundance of wonderful homeless chi's. Adding to that population for any reason other than bettering the breed is completely wrong.