i agree - talk to your local humane societies/spcas/animal control facilities about their prices and procedures. ask them what vet they use, and contact the vet to ask him/her about the procedure, and in what way the low cost procedure varies from the regular one. also, talk to other dog/cat owners you know, and find out where they had their pets altered, and what they paid.
there is no reason not to use a low cost clinic, as long as it is a good clinic and your pup has no health issues. usually the vet that performs the surgery is an area vet who either volunteers his/her services at a reduced rate or has a contract with the low cost clinic.
charges will depend on the weight of the dog. low cost spay surgery around here cost between $75 and $120 for dogs. other areas are cheaper or more expensive.
another option is spay/neuter vouchers that may be available in your area. they act as a coupon that many area vets, quite possibly your own, will honor. again, contact your local animal groups.