Food is very cheap! A coconut is around $1 and a Pad Thai as well. Remember, the more touristy places are more pricey than rural areas. This goes for anywhere in the world really.
Western Products like Pizza and Coffee is around the same price as in Europe. So you can spoil yourself once i a while. But it’s better to go with the local food to stay on a budget. Plus, the food here is amazing anyways!
The North around CHiang Mai is also cheaper than the South around the Islands. Especially Koh Lipe is expensive, but still very cheap compared to European Islands.