Flavoured waters and fizzy drinks cost a huge amount more than a glass of tap water, but you don't have to put up with entirely plain drinks to make savings — you can infuse water yourself with fruits, vegetables, herbs, and spices. It's super cheap and healthy too.
Comparison: Volvic Touch of Fruit Lemon & Lime usually costs around £1 for 1.5 litres. All you need is a few slices of a loose lemon or lime (around 30p each) to infuse your own water.
If you usually go for fizzy drinks like Coca-Cola, which can typically cost around £2 for 1.5 litres, then you'll make even bigger savings (and reap bigger health benefits, too).