Fresh berries in the supermarket.
Especially things like blackberries and blueberries - £2.00 for a small punnet, really only 1 portion.
And it's not as if they ever taste really good either - but if you go into the 'Taste the Difference' range then obviously you pay even more.
It really annoys me that if you want to eat a variety of slightly more interesting fruit, you have to pay through the nose.
Bananas are constantly cheap, you can get cheap apples and pears and (usually poor quality) oranges, but most other stuff tends to be expensive.