Normally, when I stop at one of the big burger fast food chains, I go to the desk, tell what I want and simply pay whatever the cashier asks for. But today, I just stopped for a big coke. The cashier asked for 2.59 €. Oh? What happened to the 1.99 € price we had for years? Okay, I check the price list on the wall and the big surprise is this: 500ml coke is 2.59 €. A 250ml coke is 1.10 €. You see? It's actually cheaper to buy two small ones, than one big one!
By the way, same can happen for menus: Sometimes it's cheaper to order the menu (burger, drink and fries) than burger and drink alone. Crazy if you pay for not getting fries.
And next time in supermarket, check also the prices. The big family pack is often more expensive than a few of the smaller packages combined. See also: Article in telegraph.co.uk.