The cost of living in Thailand is significantly lower than that of most western countries. Even though prices will vary slightly from place to place, in local fresh market you can find inexpensive good quality food and shopping.
Standard Thai meals at food stalls cost 30-40 Baht while meals at restaurants will range from 50-100 Baht. Western meals will usually go for 100-300 baht and western fast food restaurant will cost between 100-200 Baht.
Bottled water from a shop will cost 15 Baht for a 1.5L bottle, however water dispensers will be scattered around town that cost 1-3 baht per liter. Transportation and travel can also be very inexpensive, costing approximately 50 Baht per hundred kilometers traveled. The Thailand currency is called Baht.
Here is a comparison of common expenses in Thailand versus in the United States (all prices shown in USD).