PERIODONTIST. Regular dentist like your family dental doctor you go for regular check ups, cleaning, fillings, extraction etc.A periodontist is a specialist who only deals with diseases or gums and has special post graduate training and qualifications.
2- 3 years training. 2 - 3 years of specialty training after dental school. Cost can be as much as $150 - 200,000, depending on school, teaching position, scholarships, etc. It is a long haul.
In part. So a periodontist does 2-3 years more training than a general dentist and then limits their practice to focus on periodontal examinations, diagnosis and treatment. Yes the treatment can be more costly with a specialist, but usually the quality of the care, the extra time put into training etc are worth the increased cost.
Didactic, research. A periodontist is a specialist. His training usually contains more research and stringent didactic requirments at an accredited university. A dentist, on the other hand, is only required to attend continuing education class. There is usually a lot less research involved. Nevertheless, the cost should not reflect how much education one has, but more of the final result.