One of the great things about living in Thailand is that Thai food is not only really good, but it is also incredibly cheap and inexpensive.
Thai food usually costs the equivalent of between one and three dollars for a typical dish. At the Thai restaurant I frequent, you can get a regular sized meal for $1.50, and a double sized meal for $2.50.
Keep in mind that these prices are for Thai restaurants geared to the locals. The restaurants that target tourists are obviously going to jack their prices up.
One of the best places to get cheap and delicious Thai food here in Thailand is from the numerous food carts that pop up along the street after the sun goes down. For $1-$2 you can get a great Thai meal at these restaurants on wheels.
Of course, trendy Thai restaurants in America and other relatively expensive countries are going to be a lot more expensive than those here in Thailand.