It's right to say that how much you need will depend upon how you want to spend your money. I managed a month around Thailand and Vietnam last year on littel over £1200. That included flights to and from Vietnam from Bangkok, flight from Hanoi to Nha Trang, bus from Nha Trang to Ho Chi Min, train from Bangkok to Chaing Mai, VIP bus from Chaing Mai to Phuket, boat from Phuket to Phi Phi, transport from Phi Phi to Krabi and over to Surat Thani, from there to Koh Phangan for the full moon party, boat from Koh Phangan to Koh Tao and boat and bus from Koh Tao back to Bangkok.
I stayed mostly in hostels or beach houses which you could get for as little as £2 per night. If you want air conditioning or a fan expect to pay from about £5 upwards. As for eating, I ate a mixture of street food, restaurant food and cafe style food. I also used 7 eleven shops for stalking up on water, drinks and beers.
I feel I didn't miss out on anything on my trip and I got to do and see everything I set out to, I guess you just have to be a bit more responsible and sensible with your money when planning things. Also, don't be afraid to barter for things, even for rooms/accommodation in some places, transport costs and anything you buy from the markets etc. This saved me a lot of money!
I also met a girl on my travels who managed 4 weeks on £500 so it is more than achievable to do Thailand on a budget. Hope this helps and have a brilliant time, it is one of the most amazing places in the world!