Can low calorie be frugal?

Of course it can!

For example..you have sandwiches for lunch but you want to cut down on the calories.
If you make your own bread or buy unsliced bread, it is easy.
First,only use marg when the filling cant be spread without it.
For a salad /pate/egg/jam sandwich,you wont need any fat at all.
You dont often need fat on toast either so sometimes that is the healthier option.
Secondly, slice thinly.
If you have slices that are 4mm thick instead of 8 mm,that is half the bread and so,half the calories.
Simples

Make your own fat free pates. They are quite easy. Use cooked meat/fish or beans and put them in the blender with water to make  spreadable. If it gets a bit too runny, add some breadcrumbs.

Homemade yoghurt can be low fat too.
Use skimmed milk.
For UHT milk,heat it to blood temperature (test on your wrist like a baby bottle) in a jug,
then stir in a big spoonful of plain bought yoghurt.
Put the  jug in an insulated food bag like the kind you take picnics in or the kind you can get for shopping for frozen goods.
In 6 hours,the whole jug will be low fat yoghurt.

Want to sweeten cereal?
Use jam,especially home made.
It is made with sugar but usually half the volume is fruit and so a spoonful of jam is less calories than a spoonful of sugar but you still get the sweet taste.

You can sweeten with honey or dried fruit too but neither of those are particularly cheap at the moment.

Want a snack and there is nothing in the fruit bowl? Do you go down to the shop for biscuits or chocolate? NO Eat a crunchy carrot. They are sweetish and your teeth will feel so smooth afterwards.

Compromises....
You want chocolate but you know you shouldn't..
Buy a small plain bar and eat just a square or two.
Share the rest with everyone so there is no store to feast on.

Make your own fish and chips without the batter. Flour the fish to dry it,put on a baking parchment in the oven, cook 30 minutes. Eat with either a baked potato or home made unpeeled chips(drain well)and give yourself a sensible portion. This is nice with salad as when you are trying to keep sensible portions, it helps to fill the plate up .

Make pizza at home.
Use multi toppings that look nice (sardines and pineapple/ pepperoni and mushroom).
Cook without the cheese and then grate some on top and let it melt for a few moments.
You will get the full cheese flavour without using too much.
Make a pizza family sized and share it,don't make each person their own or you will make them too large.
If someone doesn't like a certain topping, have it as a side dish so people can add it themselves.