I just had a root canal treatment at the dental Clinic in Sukhumvit Soi 49. Very professional, good equipment and doctors studied in the US, Japan and Germany. But they are on the expensive side in Thailand. All doctors are specialized, and only do one job. So for the final filling I needed another appointment with the dentist who does the fillings. That might be a setback if you are short with time.
Dental Hospital
88/88 Sukhumvit 49
Bangkok 10110, Thailand
Tel. 02-2805000
email: [email protected]

The root canal treatment was 6000 Baht not including the x-ray and final filling.
(850 for panoramic X-ray, 1000 for the filling, and around 150 for a normal x-ray)