My Thai girl friend and I vacation in Thailand once a year and I am the cook both here and in Hawaii. We spend most of our time in her home town which is in a rural area. The closest markets are 15 minutes away so in between our trips to town I buy a variety of produce and fresh meats from the vegetable truck which does not carry junk foods. I am not in a position to compare prices, but do find the prices of junk food in 7 Eleven and Lotus are not really that cheap.
For snacks, we eat in season fruits, peanuts, dates and popcorn., along with yogurt. Today, I fried taro for my girl and her sister. Junk foods are mostly reserved while on the road.