The real issue of cost and effectiveness rests with the overall results of the service. Many pet owners benefit from traditional classes in the park. Stubborn or difficult animals might not learn under group conditions. As with most things, a little research up front may save you time and money later. If the trainer advertises behavior modification for solving problems, ask for veterinary references.
Many medical conditions mimic behavioral problems.
If the trainer does not have a good working relationship with at least one veterinarian, he might not know the difference. The best dog trainer might not be the best people trainer. If the trainer does not leave you with practical skills after the training is over, the service is of little use. Shortly after the trainer goes away your dog's skill will leave, too. To take advantage of any training course, you must practice to be proficient.