It depends a lot on what type of traveller you are and how much travelling you plan to do.
With a couple of exceptions (notably Ko Phi Phi and some parts of Phuket) you should be able to get a basic fan-cooled room for under 300 baht. If you want air-con then you're looking at ther 500-1,000B range. Facilities like swimming pools, business centres and again push the price upwards. Take a look through the various accommodation sections on Travelfish to get an idea of what standard of accommodation works for you.
If you're eating on the street, three meals a day, then 100-150B per day is adequate. If you're eating three staple meals in a backpacker style restaurant, you'll be spending more like 250-400B a day. If you're eating dinner at proper restaurants, then
you could easily be spending 300B or more a meal.
Thailand is hot and you'll need to be drinking a lot of water. At least two to three litres of water a day -- that will cost around 40-60B a day.
Booze is generally cheap compared to what you'd pay at home. A large bottle of beer can cost around 50B, a bottle of good gin or vodka around 600B. If you've having a couple of big beers every night with dinner, this will add up quickly.
Buses, trains and low cost flights are all very affordable. There is no real guideline in this regard as it depends entirely on how far and frequently you are travelling.
Ways to spend less
The following stories on Travelfish may be useful: