... i buy the box of frozen mixed berries from Aldi (very cheap)

Also i buy the vanilla cake mix from there (OMG, the raw batter is to die for) and pour it into to each muffin hole then DD loves to pop all frozen one blueberry, one raspberry & one strawberry into each muffin batter in it's tin. They don't completely rise but they are so YUMMY (especially when warm)