Like most of South East Asia, Its pretty inexpensive eating in Thailand. You can eat at the street-side eateries for 1 to 2 USD. The food is good and safe (just make sure that you find a place where the locals are buying food too).
There are many such places with a spread of local vegetables and meat dishes. Just point and choose the dish that you want. This is my regular eating place just next to Phrom Pong station in Bangkok - I pay about 60 THB for a portion of rice and 3 other side dishes.
This plate of Phad Thai (stir fried flat noodles) costs around 40 THB.
If it rains or you want a more comfortable setting, head to the food courts at shopping malls. You will spend a bit more here.Restaurants are more expensive, depending on the venue and service - prepare to spend about 150 THB and more. Bangkok is a good place to try cuisines from all over the world as there are people from all over the world living here.