Inspired by the thread about things that are inexplicably expensive!
My pick is bananas. You can buy a huge bunch of bananas for a few pence - I'm always shocked when i get to the till and find I've got about 22 bananas for less than a quid exaggerating but they've been imported from halfway round the world - yet the British apples next to them are twice as much.
What do you find inexplicably cheap?