Every layer under the ice is preserved in sugar syrup. Sweetened red beans, garbanzos, kaong, nata de coco, macapuno and minatamis na saging are all about 150 to 200 calories per 100g in their syrup. A regular glass has around 120g of them, which is 200 calories before any topping goes on. The ice is doing nothing except making it feel light.
The toppings decide whether it is 450 or 650. A slab of leche flan is 100 calories for 40g. Ube halaya is 90 for the same weight because it is mashed purple yam cooked with condensed milk and butter. A scoop of ube ice cream is 100 to 120. A carinderia halo-halo for 50 pesos skips the flan and ice cream and lands around 300. A restaurant special for 150 to 200 pesos has all three and lands past 550.
It is a dessert most people eat after a full meal. A lunch of sinigang with rice at 580 and then a special halo-halo at 560 is 1,140 calories. The 3pm merienda version on its own is easier to fit, because it replaces a snack instead of following a meal. The sugar hit is about 70g either way, roughly the same as two cans of Coke.