There's loads of variables that need to be put into perspective before you can figure out how much you need to budget for.
- Type of accomodation - Prices vary everywhere. For budget accomodation I paid 70baht (£1) in Kanchanaburi but 600baht in Phi Phi. My accomodation at Koh Pha Ngan cost me 150baht and was the epitome of basic but more than adequate for my needs. Depends what creature comforts you need. You get what you pay for and I paid for a small shed with no electricity but a hammock

- Drink

- I like a drink, especially when I'm on holiday when I like a lot! Luckily they are reasonable in Thailand where 60 baht will get you a large Chang in most places. 150 baht will get you a SangSom bucket on KPN! I can't say that I budgeted at all when I was in Asia and just drank when I wanted so you may want to think about it as it could add up quickly! Maybe £10+/day to be safe lol!
- Travel - How quickly do you get bored in 1 place? You'll probably want to move around the country and that costs money. 500-1000 baht will get you from Bangkok to the islands/Chiang Mai. Smaller distances will of course cost less. You could of course take a local bus and pay even less. You may choose to fly at some stage so maybe factor that in as well.
- Activities - I'm normally a bit of a lazy bastard but travelling I seem to want to do a lot of activities. Think about any activities you might do (trekking, tubing, kayaking etc) and add more to your budget.