It's very cheap, (although slightly more expensive than neighboring countries of Laos, and Cambodia.)
Food. This typically ranges from 60 to 200 baht depending on your dish in a restaurant (1.5 - 6 USD). You can get very cheap Pad Thai from street vendors for 30 baht (80 cents). Beer from a shop is about 30 baht for a small bottle.
Accommodation. Expect to pay between 1200 and 2000 baht per night for a decent hotel room in Bangkok. Hostels and bungalows on the islands range from 500 to 800 baht. At the moment many hotels are offering the low-season rate since bookings are apparently down as much as 70% in some places. The best thing to do is walk around to find a cheap / nice place. I've also used wotif.com to get cheap bookings.
Transport. Taxis in the cities are the best way, although be sure to confirm the price beforehand and always ask to go 'by meter' as it's always the cheapest way. Many people use tuk-tuks (small three wheeled bikes) but these can be very dangerous as they drive like crazy! You travel around the country by bus or overnight trains, and nearly all hotels and travel agents can help you book. about 10 years ago
Ian's answer is good; I have maybe a few additions. If you want western food expect to pay more. Cheapest around 200 baht up to 1,000. The cost depends on your selection and where you go. For a small beer, you will get a can, not a bottle, for yes, 25-29 baht. For hotels it is possible to be a little cheaper, maybe around 800 baht and still stay in an OK place. Taxis is easy, but local bus or songtow is much cheaper. The trick is getting the right one and getting out at the right place. It depends on how much of an adventurer you are. over 7 years ago