Expenditure depends where are you visiting in Thailand. If you visit Pattaya and Bangkok, it is different and if you visit Phuket and alike island, it can be different. Plan your destination first. Secondly, expenditure always depends on your travelling style. For example, I don't like staying in Hotels, I always stays in Hostels because I can meet different Nationals there. I hate eating food in classy cafes and restaurants. I stick to local food or street food. I can give you an estimate that I have visited Pattaya and Bangkok for 25000 with flights for 6 days.
The expenses will completely depend on the places you're planning to visit, your accommodation and activities. Thailand has all sorts of options from budget hostels and hotels to high end luxurious staying - so you can decide and plan your budget accordingly! Also, the flight costs vary - in case you're flying from Kolkata or Chennai, you'll get cheaper tickets for Bangkok, than Delhi or Bombay!
I've been to Thailand twice, and if you ask me, the overall expenses weren't much! Recently I traveled to Bangkok, Krabi and Koh Phi Phi with a friend for 6 days, and we spend around 35K INR per person, including flights, speedboat rides, etc and also, we stayed at beachfront hotels.