Don't worry about going to the humane society. You definitely want to double check, but ours does pre-surgery blood work and ONLY use isofluerene on ALL breeds (including Yorkies). They will usually pull leftover baby teeth for free while your dog is still under. Spay/neuter clinics are usually even cheaper. In Chicago they do it by zip code. My zip code pays $40, but the one next to mine pays nothing because the median income is lower. You definitely want to call around and make sure your baby gets the best care possible.
Also, I disagree w/ making the dog stay over night. Stewie had major surgery and the vet let him come home the same night -- 12 hours after the procedure. I don't think that a simple neuter or spay would require an overnight stay, let alone two nights, but again I am no vet. You should do whatever you're most comfortable with
Don't be embarassed at all for considering the humane society or a shelter - that's what they're there for!