Earlier in the month I wanted to go to the Nittaya Dental Clinic, by the bus station, 296/49 Saiutit Road (about 3 minutes walk from the flower stall end of Robinsons (watch the slippery pavement)) to check out their prices and have a clean and polish. Unfortunately went on a Wednesday, the only day they are closed
I found another Dentists Just across the road, opposite side to the bus station. The receptionists didn't speak much english, but the dentist did. She had a pricelist in a book and I made a note of some of the prices, but can't find it!! From memory though the price you mention seems at the top end of any of the treatments on the price list and for implants it may have been closer to 30,000?? The ultrasonic claen and polish (FAR better job than they do in the UK, proper sparkling!) was 400-600baht in the book, I ended up paying 600baht. The service, equipment and dentist were far better than any I have been to in England.
However, from my experience, the pricelist book was all in english and I suspect that they have another pricelist book in Thai which may contain different prices (at Don Muang the Salon there had 2 books and the prices were considerably different). Maybe you can get a Thai to enquire of the dentist the costs, for a friend, and then pay a visit after?