There is a medical condition called cryptorchidism where one or both testicles do not descend into the scrotum. In a fetus the testicles develop near the kidneys and during and shortly after pregnancy they move through the abdomen, through the groin, under the skin, and down into the scrotum. At any point along that journey a testicle can stop moving and remain where it is. This can be a problem since a testicle is not designed to stay in the abdomen, and the increased body temperature can predispose it to become cancerous over time. Why? Testicles in mammals are actually designed to function best at a little less than body temperature, which is why they are normally housed outside of the core of the body.
By a few weeks old both testicles should be noticeable on a routine exam, and definitely by 3-4 months old. If they haven't descended into the scrotum by then, they really aren't going to. This means surgery, and not just a routine one. If the retained testicle is under the skin next to the penis, it's usually just a matter of making a second incision and taking it out. But if the testicle is retained in the abdomen, it can be quite a challenge to find it. We have to make an abdominal incision and go hunting for it. It's a little like doing a spay on a female, but the testicle is smaller than a uterus and often harder to find.
Why bring this up? Well, many people aren't aware of this condition. If your vet diagnoses your dog with this disorder, follow their recommendations and have your dog neutered and the retained testicle removed. And when they present you with the extra charges, especially if it's in the abdomen, understand that this is often more difficult than doing a spay and the fees are necessary. One of my associates was doing this surgery yesterday and had a heck of a time finding that retained testicle. It took about 45 minutes, when a routine neuter for her is less than 10 minutes and a routine spay 30 minutes or less. That poor puppy had a very large abdominal incision, but thankfully he will heal and be just fine. But it was a reminder that this kind of surgery isn't always routine.