The problem is that you do have some unscrupulous vets that take advantage of unsuspecting customers. Take a look at this expose: http://www.knbc.com/news/15377166/detail.html
While I'm not a vet, I've been around dogs doing dog rescue long enough that I am at least aware of what needs to be done (and not done) for routine things that come up (lose stool, kennel cough, etc.) and I've still had vets try to take me to the cleaners.
In the end, the best way to prevent being taken advantage of is to get as much information as possible, ask a lot of questions, do research and even get a second opinion. If the vet you are using is reputable, they should welcome you wanting to make sure that their recommendation is the best one. You really have to be your pet's advocate. And don't be afraid, if you don't like the vet you're using, to shop around. This is the person that is going to make your babies well and maybe one day assist in sending them to the Rainbow Bridge; they are important and you owe it to yourself and your animals to find one that you are all comfortable and happy with.