From Travel Agent in the Ko San Road are of Bangkok. Cheap tickets to various points in India prices are in US dollars and are listed ONEWAY and then RETURN fares. Usually it best to arrive in Bangkok before booking a ticket to India. Shop around the various agent in Bangkok to see the best deals. Gernally Bangkok to Kolkotta is the cheapest and shortest route.
Flys Bangkok-Kolkotta, Kulla Lampur - Chenni for about $125.00. From KL you can take the bus to southern Thailand