We were in Thailand for a total of 39 days and spent a total of $3850, for an average of $98.74 per day.
We averaged about $38 per night on accommodation staying mostly in flashpacker hostels, though there were a few grungy places in there (Seabreeze in Koh Samet, I’m looking in your direction). If you wanted, you could spend MUCH less than this (a bed in a a dorm of a nice hostel will run you no more than $10).
Food here in unbelievably cheap and delicious. In Bangkok there are vendors on literally every street selling everything from fruit, to thai omlette, to pad thai, to untold different varieties of soup. Each of these things will cost no more than about $1 for a big serving of something amazingly tasty and awesome. You will NEVER find a bad meal on the street. A large beer will cost about $1.50 from 7-11.
Internal transportation was relatively inexpensive. The overnight train from Bangkok to Chiang Mai was a total of $54 and the return trip by plane was $150. Buses and minivans never ran more than $10-15.
Normally, US citizens can enter Thailand for free but since we would be here longer than 30 days we needed 60 day tourist visas, which cost $100.
Our one big splurge here — We spent $160 for our day playing with the elephants at Elephant Nature Park. Just to see the 3 month old baby elephant, Navaan, it would have been a bargain at twice the price.