It is a well known fact it is cheap to eat street food in Thailand.
However, if you decide to eat at a mall or western style food, you might be surprised with the high cost.
Shown as an example, a set at Pizza Company is 599 thb or ~US $17.50. Compared to 40 baht dishes of Thai Street Food, that is quite expensive.
KFC has a All in One Bucket of chicken that is more reasonable at 299 thb (US $8.79)
McDonalds and Burger King are just as, if not more expensive than in the west.
Westerners/Expats may find these prices quite reasonable, as they are easily able to afford them.
For the average Thai, this cost would have to be split 4 or 5 ways.
You can find just about anything you want to eat in Thailand, it is all dependent on costs that you can afford.
Western fast food franchises have popped up all over Thailand:
McDonalds
KFC
Burger King
Subway
Starbucks
Dunkin Donuts
Hooters and more!