My local Asda has Rubicon soft drinks cans 2 x pack of 6 for £3, this offer was for the passion fruit flavour, definately the best, not sure about the other flavours.
Not online though, I did check. Online you can get 4 cans for £1.50 which is not great as they cost about 40p each anyway. However you can get the 2l bottle of Mango drink for 2 for £2. Also if you like the guava berry juice the small carton is only 10p.