Often things can go on offer (say with £1 off), making it look really appealing, but right next to it could be a larger version of it which actually works out cheaper when comparing the prices per volume.
Here's an example, Asda currently have 340g Whole Earth peanut butter on offer/rollback for £2.00...
However, they also sell a 1kg pack of the exact same peanut butter for £5.85
This actually works out cheaper per 100g (in this instance by 0.3p admittedly, but often it can be more). This is just to show how offers may not be as good as they seem!
Things to consider are:
- Do you need the larger quantity?? Or overall due to your usage will you save by buying the more expensive per 100g.
- Can you eat/use the larger quantity before it goes off?