haha! it's essentially a slang phrase that friends may say to each other- "you're such a trip!" meaning "You're crazy!" (crazy in this context meaning funny). Though to be honest it seems like a phrase for generations age 25-35ish. But it may just be me.

The only difference would be that using the word "up" comes across more friendly, as a family member or close friend might say. Without it, it's still very friendly, but it may be how a boss or school teacher calls to see how you are doing.

Let me use them both in a conversation: Two friends watch a very beautiful girl leave the building they are in. "Do you think we'll ever see her again?" "Don't bet on it." Meaning it is very unlikely. 2. "Do you think I could fly like Superman?" "No, it's impossible." Meaning it will never happen.

It's more of a statement used when something is given to another, even more specifically if there are very few of this item in existence. For example, your grandfather gives you $100, and tells you to spend it well. The correct expression would be "When it's gone, it's gone.". The statement after the comma is just confirming that it will indeed be gone, when you lose it/spend it/ etc. It's not a very common expression.