It's short for "mind you", which is itself short for "(May/ can) I remind you" - it is similar to "Don't forget..." "You/ we have to remember...", "You need to keep in mind that ..." You find "mind you" at the beginning or at the end of a sentence (or sometimes even in the middle.) It's probably more common at the beginning. For spoken and informal English... - I've got a feeling it's a bit more expensive, mind (you). - Mind you, I've got a feeling it's a bit more expensive. - I've got a feeling, mind you, that it's a bit more expensive.