As you travel around Thailand, you will see a lot of roadside peddlers selling coffee and tea. They sell the commercial Nescafe as well as the old version of coffee- that existed long before important coffee came by.
A cup of the coffee/ tea cost between 10 to 20 baht. I normally buy from these peddlers for my caffeine fix. With evaporated and condensed milk, they taste much nicer than those canned coffee from the stores.
I bought from this peddler and asked him permission to take a photo- this photo was taken in Nakhom Pathom, a town about 2 hours drive from Bangkok.
Aside from that, you can also get lattes and expressos for relatively low price from little stalls that is sometimes set up outside shops. A cup of hot latte may cost you only about 25 to 35 baht (less than USD1).