Hello, this is Dr. Benedetta, and I would be glad to help.
The cost of surgery will vary depending on whether a regular veterinarian will do it, or a boarded surgeon, and on the area you live in.
However, for this kind of surgery, it is best to wait until the dog is at least 1 year old, because the anatomy will change as she grows, and she may even grow out of this condition, and not need a vulvoplasty. After first heat also the chance of needing a vulvopasty are fewer
Also, this surgery can be combined with spay surgery, which would make sense because then she has to go under general anesthesia only once, and anesthesia is a big part of the cost of surgery.
Overall, i'd expect to be paying at least $1000, probably considerably more, but again it will vary.
You should ask for surgical estimates before going into this, but be aware that most veterinarians and veterinary surgeons will want to see the patient and you'll have to pay for a consult.