Coconut water alone is light. About 19 calories per 100ml, so the 300ml in a cup is 57 calories. The soft young buko meat is about 80 calories per 100g, and a cup gets 50g of strips for 40 calories. That is the honest coconut, about 100 calories. Then the stand adds simple syrup, usually 15 to 20g of sugar, because plain buko water tastes flat to people used to soft drinks. That is another 60 to 80 calories.
The 'buko juice' in a plastic cup at a mall or a Buko Nation type stall is often not straight from a coconut. It is diluted coconut water with sugar, sometimes condensed milk, and more buko strips, and a large cup can hit 250 calories. The bottled versions at the supermarket run 60 to 90 calories per 330ml, which is less than soft drinks but still 15 to 20g of sugar.
Buko juice is a good replacement for Coke (140 calories) and an even better one if you ask for 'walang asukal' (no sugar), which vendors will do. It is not a free drink though. Two cups of the sweetened kind in an afternoon is 320 calories, which people do not count because it is 'just coconut'. The potassium claim is real, about 250mg per 100ml, useful after a hot day.