It depends on how many nights you wish to stay in Thailand and in which cities. Cost wise Bangkok and Pattaya are the cheapest. Pattaya is a 2 hour drive from Bangkok. So if you're looking for a metro feel, cheap food and shopping, high-rises, lots of traffic, and some guy fun (read: bars, streetwalkers, etc), then 5 nights should be enough and you can cover both these cities in around Rs. 50,000 in good hotels, including air travel cost ex-Mumbai. But if you want to visit the beach - want an island feel - and sea view rooms and Thai massages, then you can go to Phuket, Krabi or Koh Samui. Krabi is more expensive than Phuket and Koh Samui is more expensive than Krabi, or at least that's what I've heard. Because you have to travel to Phuket/Krabi from or via Bangkok, so the airfare increases. Krabi is 4 hours away from Phuket by road, so you can cover both these islands in one go. Koh Samui is far, that's why, again, the airfare increase. So 5-6 nights in these two cities should cost around Rs. 60-65,000 per person, depending upon the hotels and the air carrier (ex-Mumbai) you choose.
In my experience, the perfect package would include 3 nights in Phuket (there's quite a lot to see and do in Phuket - you can visit Phi Phi Island and James Bond Island or do the elephant safari), 2 nights in Pattaya (there's the Sri Racha Zoo, Alcazar Show, Art in Paradise museum - if you're going with friend/family - then there's Nong Nooch Village enroute to Bangkok) and then 3 nights in Bangkok (you can visit Safari World Marine Park, visit the Buddha temples, go shopping, Dream World if you want amusement and rides, etc). The entire package should cost you approx. Rs. 70-75,000 per person... or cheaper if you book lower than 3 star hotels and find some kind of awesome online deal.
P.S. I mentioned ex-Mumbai twice because flights from Mumbai, Ahmedabad and South India have differing rates. If you're catching an ex-BKK flight from, say, Chennai, it will cost you less than if you book the flight from Mumbai. The approximate rates that I mentioned above are all ex-Mumbai.